Exhibit 99.1

Subsequent events included in Note 8

On behalf of the Board of Directors:

Supplemental cash flow information (Note 14).

OceanaGold Corporation (the “Company” or “OceanaGold”) is domiciled in British Columbia, Canada and the registered address of the Company is Suite 1020, 400 Burrard Street, Vancouver, British Columbia, V6C 3A6, Canada. The Company’s common shares trade under the symbol 'OGC' on the Toronto Stock Exchange (“TSX”) in Canada and, as of April 7, 2026, on the New York Stock Exchange (“NYSE”) in the United States.

The Company is engaged in the exploration, development and operation of gold and gold/copper mines. OceanaGold operates four operating mines: the wholly-owned Haile Gold Mine in the United States of America; the wholly-owned Macraes and Waihi operations in New Zealand; and the 80%-owned Didipio Mine in the Philippines.

These unaudited condensed interim consolidated financial statements were approved by the Board of Directors on August 5, 2026.

These unaudited condensed interim consolidated financial statements have been prepared in accordance with International Financial Reporting Standards as issued by the International Accounting Standards Board (“IFRS Accounting Standards”), as applicable to the preparation of condensed interim consolidated financial statements including IAS 34. Accordingly, certain disclosures included in the annual financial statements prepared in accordance with IFRS Accounting Standards have been condensed or omitted. These unaudited condensed interim consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements for the year ended December 31, 2025.

The accounting policies applied in the preparation of these unaudited condensed interim consolidated financial statements are consistent with those applied and disclosed in the Company’s audited consolidated financial statements for the year ended December 31, 2025. The Company’s interim results are not necessarily indicative of its results for a full year.

The Company adopted amendments to IFRS 9: Financial Instruments and IFRS 7: Financial Instruments: Disclosures effective January 1, 2026. The Company has elected to apply the amendment in IFRS 9 relating to the derecognition of financial liabilities settled through electronic payment systems. The adoption of these amendments did not have a material impact on the Company's financial statements.

New IFRS accounting standards and pronouncements - not yet adopted

IFRS 18: Presentation and Disclosure in Financial Statements

In April 2024, the IASB issued IFRS 18: Presentation and Disclosure of Financial Statements ("IFRS 18"), which replaces IAS 1: Presentation of Financial Statements. IFRS 18 introduces a specified structure for the income statement by requiring income and expenses to be presented into the three defined categories of operating, investing and financing, and by specifying certain defined totals and subtotals. Where company-specific measures related to the income statement are provided, IFRS 18 requires companies to disclose explanations around these measures, which are referred to as management-defined performance measures. IFRS 18 also provides additional guidance on principles of aggregation and disaggregation which apply to the primary financial statements and the notes. IFRS 18 will not affect the recognition and measurement of items in the financial statements, nor will it affect which items are classified in other comprehensive income and how these items are classified.

The standard is effective for reporting periods beginning on or after January 1, 2027, including for interim financial statements. Retrospective application is required. The Company is currently assessing the effect of this new standard on our financial statements.

The Company makes estimates and assumptions concerning the future. The resulting accounting estimates will, by definition, seldom equal the related actual results. Areas of estimation and judgement that have the most significant effect on the amounts recognized in these financial statements are disclosed annually, and were last disclosed in Note 4 of the Company's consolidated financial statements for the year ended December 31, 2025.

The following table summarizes the net book value of mineral properties, plant and equipment as at June 30, 2026 and the changes during the period then ended:

Authorized capital

The Company is authorized to issue an unlimited number of common shares with no par value.

Share Consolidation

On June 23, 2025, a share consolidation was completed on the basis of one post-consolidation common share for every 3 pre-consolidation common shares. The consolidation reduced the number of common shares issued and outstanding from 693 million common shares to 231 million common shares. The common shares commenced trading on the Toronto Stock Exchange (the "TSX") on a post-consolidation basis on the opening of trading on June 23, 2025. The number of shares issuable under the Company's stock-based compensation plans were proportionately adjusted upon completion of the consolidation. All information relating to earnings per share, issued and outstanding common shares, share rights, deferred units, and per share amounts in these financial statements have been adjusted retrospectively to reflect the share consolidation.

Dividends

During the quarter ended June 30, 2026, the Company declared and paid a dividend of $0.09 per common share totaling $20.0 million to equity holders of the Company (quarter ended June 30, 2025: declared and paid a dividend of $0.03 per common share and paid the declared dividend of $0.03 per common share from the quarter ended March 31, 2025, totaling $14.0 million year to date to equity holders of the Company).

On August 5, 2026, OceanaGold declared a dividend of $0.09 per common share.

Share buyback

In July 2026, the Company received approval from the TSX to renew the NCIB, permitting the Company to buy back up to 22 million common shares in the open market through the facilities of the TSX, NYSE and alternative trading systems in Canada and the United States during the period commencing on July 24, 2026 and ending on or before July 23, 2027.

In connection with the NCIB, the Company renewed its automatic securities purchase plan ("ASPP"), which allows for the purchase of common shares under the NCIB at times when the Company would ordinarily not be permitted to purchase shares due to regulatory restrictions and customary self-imposed blackouts. As at June 30, 2026, no obligation to purchase shares (December 31, 2025: nil) was recognized under the ASPP.

In 2025, the Company completed the maximum $175 million in share buybacks under the Board of Directors approved share repurchase program for that year.

On February 18, 2026, the Board of Directors approved additional share buybacks to a maximum of $350 million of common shares in the open market through the facilities of the TSX, NYSE and alternative trading systems in Canada and the United States under the NCIB program. During the second quarter of 2026, the Company repurchased and cancelled 1.8 million common shares for consideration of $57.5 million at an average price of CAD$43.94 per share.

Performance share rights plan

The following table summarizes the outstanding rights granted under the performance share rights plan as at June 30, 2026 and December 31, 2025 and the changes during the periods then ended:

The performance share rights outstanding at June 30, 2026 had a weighted average remaining life of 1.7 years with no exercise price.

Performance share rights granted to designated participants may from time to time vest when the Company meets target milestones for the applicable performance period, in accordance with the vesting schedule established at the time of grant by the Board. There are two components to each performance share right: a performance condition based on the Company's share price performance relative to peers ("TSR") and a service condition. Rights granted were priced using a Monte Carlo simulation to model the Company’s future price and TSR performance against the comparator group at the future vesting date. The performance condition weighting varies according to the designated participants’ job levels with vesting up to 200% of target. Upon vesting at discretion of the Board, the performance share rights are payable partly in shares and partly in cash in accordance with the plan.

In 2026, performance rights vested at 133% (2025: 156%).

At June 30, 2026, the fair value of the outstanding unvested performance rights and corresponding liability of $40.4 million (December 31, 2025: $62.9 million) is recorded within employee benefits.

Cash rights plan

Under the cash rights plan, the Company issues cash rights to certain employees that require a cash settlement linked to the prevailing share price of the Company after completion of the three-year service based vesting period. Any changes in fair value are recognized in the Condensed Interim Consolidated Statements of Income with a corresponding increase or decrease in liability (2026: $14.1 million, 2025: $21.5 million).

Deferred Unit Plan ("DUP")

The following table summarizes the outstanding deferred units granted to non-executive directors under the deferred unit plan as at June 30, 2026 and December 31, 2025 and the changes during the periods then ended:

The fair value of the units granted under the DUP is calculated as the estimated future cash flow and it is remeasured at each reporting date and at the date of settlement. Any changes in fair value are recognized in the Condensed Interim Consolidated Statements of Income with a corresponding increase or decrease in liability. At June 30, 2026, the fair value of the units and corresponding liability was $11.0 million (December 31, 2025: $12.1 million) at a share price of $25.08 (CAD$35.55).

OceanaGold Philippines, Inc. ("OGPI") holds the Company's interest in the Didipio Mine. OGPI is listed on the Philippine Stock Exchange ("PSE") with 20% of its common shares held by public shareholders as required under the terms of the renewed Financial or Technical Assistance Agreement ("FTAA").

The following table summarizes the balance sheet of OGPI:

The following table summarizes the share of profit attributable to the non-controlling interest:

1. All concentrate sales are generated by the Didipio mine.

Under the FTAA, “Net Revenue” is the gross mining revenues derived from operations, less allowable deductions and an amortization deduction.

Allowable Deductions under the FTAA include expenses attributed to exploration, development and production which includes, expenses relating to mining, processing, exploration, capitalised pre-stripping, royalties, rehabilitation, marketing, administration, community and social development, depreciation and amortization and interest charged on borrowings.

All taxes and fees payable to the Philippines Government, including corporate income tax and indirect taxes such as excise, local business, property and withholding taxes as well as amounts accrued under the free-carried interest, are deducted from the Government’s 60% share of Net Revenue to arrive at any Additional Government Share payable.

The Additional Government Share accrued for the period ended June 30, 2026 of $31.8 million (June 30, 2025: $17.7 million) is recorded within trade and other payables. The Company made an Additional Government Share payment of $37.2 million in April 2026 related to 2025 amounts accrued at December 31, 2025.

1 Includes changes in rehabilitation provisions and other non-trade working capital balances.

Operating results of operating segments are reviewed by the Company’s chief operating decision maker ("CODM") to make decisions about resources to be allocated to the segments and to assess their performance. The Company’s general corporate administration costs are included within ‘Corporate and other’ to reconcile the reportable segments to the consolidated financial statements. Significant information relating to the Company's reporting operating segments is as follows:

Capital commitments

The Company has certain capital commitments principally relating to the purchase or lease of property, plant and equipment at Macraes, Waihi and Haile, and the development of mining assets at Macraes, Waihi and Didipio.

The following table summarizes the capital commitments contracted for but not provided for as at June 30, 2026 and December 31, 2025:

Year to date related party transactions solely comprise of key management personnel compensation, including salaries, short-term incentives, directors' fees and share-based payments of $13 million.

In a complaint dated July 4, 2008 before the Philippines Regional Trial Court ("RTC"), a third-party, Mr. Liggayu, disputed the terms of the Addendum Agreement and the rights of the Gonzales Group to claim an interest in the Didipio Mine. Mr. Liggayu alleged that he is the true and beneficial owner and real-party-in-interest of the Didipio mining claims, and sought to enjoin the Company from making any payments to, or in dealing with, the Gonzales Group, and instead to recognize his rights.

In a decision dated March 11, 2025, the RTC declared that Mr. Liggayu and the heirs of Mr. Gonzales Sr. are partners on a 50-50 basis, to all the rights, participation and interests, as claim owners of the Didipio mining claims in the name of Mr. Gonzales Sr., beginning January 2007 onwards. It further declared that the rights and entitlements of Mr. Liggayu cannot be directly enforced by him against OceanaGold in the existing agreements, specifically the FTAA, which Mr. Liggayu can internally claim and enforce only against the heirs of Mr. Gonzales Sr, and vice versa, (the “March 2025 Decision”).

On April 2, 2025, Mr. Liggayu moved for partial reconsideration of this decision, claiming that, among others, he is the true and lawful owner of the Didipio mining claims; if a partnership exists, it should be from 1985 and should cover all the subject mining claims and not just for Mr. Gonzales Sr’s portion of the claim; and his rights and entitlements should be directly enforceable by him against OceanaGold. Both the Gonzales Group and OceanaGold filed an opposition to Mr. Liggayu’s partial motion for reconsideration. In a decision dated October 21, 2025, the RTC denied Mr. Liggayu’s motion for partial reconsideration (the “October 2025 Resolution”). Mr. Liggayu has now filed a Notice of Appeal to each of the March 2025 Decision and October 2025 Resolution. This case is now elevated to the Court of Appeals.

The Company believes there is no near-term impact on its business or operations as the decisions do not require payment of money by OceanaGold and the Arbitration proceeding is yet to be resolved.

As of June 30, 2026, the Company has accrued $95.2 million ($74.5 million of royalties and $20.7 million related to free-carried interest) pertaining to the Addendum Agreement (December 31, 2025: $83.6 million).

Notwithstanding the fact that the Supreme Court has previously upheld the constitutionality of the Mining Act and FTAAs, the Company is mindful that litigation is an inherently uncertain process and the outcome of the case may adversely affect the operation and financial position of the Company.

The Group has also issued bonds in favour of Otago Regional Council, Dunedin City Council, Waitaki District Council, West Coast Regional Council, Buller District Council and Department of Conservation in New Zealand as a condition for the grant of water rights and/or resource consents, and rights of access for the Macraes Gold Mine and the former Globe Progress Mine at the Reefton Restoration Project which amount to approximately $47.0 million (December 31, 2025: $34.3 million). Cash payments on bonds issued to New Zealand authorities would only be paid if the Company did not meet its obligations.

The remaining estimated financial assurance of $8.6 million will be paid over the life of the mine with estimated annual assurance payments of $1.8 million to occur from 2026 to 2028, $1.2 million in 2029, and $1.0 million from 2030 to 2031. The timing and amounts of these payments could change due to a number of factors, including changes in regulatory requirements, changes in scope and timing of closure activities. The State of South Carolina in the United States of America requires financial assurance for the estimated costs of mine reclamation and closure, including groundwater quality protection programs.

The surety bond and other financial assurance must be maintained in force continuously throughout the life of the mining operation and may only be released, partially or in full, after the State of South Carolina approves its release.

Subsequent to the filing of the petition, the RTC denied the Petitioners' application for a Temporary Environmental Protection Order against OceanaGold. Further, in a resolution dated April 2, 2025, the RTC dismissed most of the issues raised by the Petitioners but decided that the issue of whether the Company’s subsidiary is currently engaged in open pit mining is a question of fact that should be decided at trial. The Petitioners filed for a Motion for Partial Reconsideration which was subsequently denied by the RTC in October 2025. The Petitioners then filed a Petition for Review before the Supreme Court, seeking a review of the RTC’s decisions. In an Order dated December 5, 2025, the RTC deferred further proceedings until the Supreme Court issues a resolution on the Petition for Review.