OceanaGold : 2024 Basis of Preparation and Reporting Indexes

SUSTAINABILITY REPORT BASIS OF PREPARATION AND REPORTING INDEX

2024

Contents

About this Document 3

Health and Safety 4

People and Culture 8

Community and Social Performance 12

Socio-economic Contribution 14

Environment 18

Climate 23

Tailings Management 29

Sustainability Reporting Indexes 30

About this Document

This Basis of Preparation and Reporting Index outlines the definitions, methodologies, boundaries, assumptions and exclusions used by OceanaGold Corporation (OGC) to calculate key sustainability metrics across environmental, social, and governance (ESG) dimensions for OceanaGold's 2024 Sustainability Report and related ESG performance data contained withing OcenaGold's 2024 disclosures. The purpose of this document is to ensure transparency, auditability, and consistency in the measurement and reporting of ESG performance data. Updates for 2024 reporting period have been incorporated to reflect improvements in methodology and data quality.

• Waihi Operation, our wholly owned underground operation, which includes the proposed. Waihi North Project, including the Wharekirauponga Underground development project ("WUG"), located in the North Island, New Zealand ("Waihi" or the "Waihi Operation")

  OceanaGold's corporate and administrative offices:

• Vancouver, British Colombia, Canada (head office)

• Brisbane, Queensland, Australia

• Melbourne, Victoria, Australia

• Manila, Philippines

• Dunedin, New Zealand

TABLE 2 - 2024 unit conversions

Pounds (lb) are converted to Kilograms using 1 lb = 0.453592 kg

Natural Gas: 1 Dekatherm = 1.0550559 GJ

Unit Converter (unitconverters.net)

Unit Converter (unitconverters.net)

REPORTING SCOPE AND BOUNDARY

This document outlines the basis on which the data for OceanaGold's 2024 Sustainability Report and 2024 Environmental, Social and Governance (ESG) performance data has been generated and disclosed for the reporting period January 1, 2024 to December 31, 2024.

Unless otherwise stated, references to "OceanaGold", "the Company", "we" and "our" in this document refer to OceanaGold Corporation and our subsidiaries.

Organizational boundary

Unless otherwise stated, OceanaGold's basis and methodology for calculating OceanaGold's 2024 Sustainability Report and 2024 ESG performance data are calculated within OceanaGold's Organizational boundary. OceanaGold's organizational boundary includes the mining operations we own and control and our main corporate and administrative offices, during the reporting period.

OceanaGold's operating assets:

• Haile Gold Mine, our wholly owned open pit and underground operation located near Kershaw, South Carolina, United States ("Haile" or "Haile Gold Mine").

• Didipio Mine, our 80%-owned underground gold and copper mine and surface stockpile operation located in Luzon, Philippines ("Didipio" or "Didipio Mine").

• Macraes Operation, our wholly owned open pit and underground operation located in the South Island, New Zealand ("Macraes" or the "Macraes Operation").

• Singapore

UNIT CONVERSIONS

Exchange rate summary

Unless otherwise stated, all financial figures reported have been converted to U.S. dollars (USD) using the weighted average exchange rates in Table 1.

Unless otherwise stated, financial figures exclude all indirect local taxes from the reported value including Goods and Services Tax (GST) and Value Added Tax (VAT).

TABLE 1 - 2024 weighted average exchange rates

AUD/USD	0.66
NZD/USD	0.61
PHP/USD	0.0175
GBP/USD	1.28
SGD/USD	0.75
CAD/USD	0.73

CURRENCIES CONVERSION RATES

Exchange rate summary

Unless otherwise stated units have been converted using the conversions in Table 2.

1 Gallon = 3.78541 Liters Unit Converter

(unitconverters.net)

RESTATEMENTS OF INFORMATION

Restatements ensure consistency and enable comparability of information between reporting periods. Restatements are also provided in cases of significant changes in reporting parameters (e.g., mergers, acquisitions, divestments). Our reported information may include restatements of information from previous reporting periods to account for changes in measurement, methodologies and/or calculations or to correct an error in previously reported information that may have a potential impact to decision useful information. Restatements related to information contained within the applicable disclosures are included as footnotes where applicable and/or outlined in this 2024 Sustainability Report Basis of Preparation and Reporting Index.

JUDGEMENTS AND MEASUREMENT UNCERTAINTIES

We are committed to transparency and robust governance practices, especially in addressing legal and regulatory challenges. We may be, from time to time, involved in various legal proceedings and claims arising in the ordinary course of business. We cannot predict with reasonable certainty the likelihood or outcome of these matters. We disclose information related to our material ongoing legal proceedings and regulatory actions as well as associated risks, in our Annual Information Form, available on SEDAR+ at https://www.sedarplus.com under our name and on our website.

Kilowatt Hours are converted to

Unit Converter

Gigajoules using 1 kWh = 0.0036 GJ (unitconverters.net)

Megawatt Hours are converted to

Unit Converter

Kilowatt Hours using 1 MWh = 1000 kWh (unitconverters.net)

Liquid Petroleum Gas (LPG) (kg) = 1.862 L (NZ)

Genesis Energy safety data sheet

1 acre = 0.40468564 hectares

Unit Converter

(unitconverters.net)

Health and Safety

MEASUREMENT BASIS

Key term DEFINITION/BASIS

Lost Time Injury (LTI)

Any injury or illness that results in one or more shifts away from work, excluding the day of the incident. Includes, Medically Treated Injuries (MTIs), Restricted Work Injuries (RWIs). Lost Time Injuries (LTIs) and fatalities.

• An injury is classified as an LTI if the worker is unable to return to work on the next calendar day regardless of whether they were scheduled to work that day.

• LTI days are counted from the first day after the occurrence of the injury where the worker has not been able to return to work.

• An injury must have a medical certificate from a registered medical practitioner to be classified as an LTI.

• A recurrence of the same injury is not classified as a new LTI, provided the recurrence is not caused by a new incident; however, an aggravation caused by a new incident shall be classified as a new LTI.

Body part injured Refers to the part of the body that was injured. Including but not limited to, head, neck, trunk, upper limbs including hands, lower limbs including feet.

Fatality A significant irreversible health effect with reduced life expectancy.

Frequency rate All rates are calculated using 200,000 hours worked:

• LTIFR: Lost Time Injury Frequency Rate = number of lost time injuries recorded per 200,000 exposure hours worked.

• RWIFR: Restricted Work Injury Frequency Rate = number of restricted work injuries recorded per million exposure hours worked. Total number of restricted work cases x 200,000/hours worked.

• MTIFR: Medical Treatment Injury Frequency Rate = number of medical treated injuries recorded per million exposure hours worked. Total number of medical treated injuries x 200,000/hours worked.

High potential incident Any potential or actual consequence Level 4 or 5 safety event.

Injury incident category definitions Category 1 incident - First aid. Reversible health effects of little concern including precautionary restricted work and clinical referral but no medical treatment.

Category 2 incident - Medical treatment undertaken, reversible health effects, LTI up to 1 week, RWI up to 2 weeks. Category 3 incident - LTI between 1-5 weeks, reversible health impacts of concern, RWI greater than 2 weeks.

Category 4 incident - Permanent/irreversible disabling illness, injury or health impact, LTIs greater than 5 weeks. Category 5 incident - Fatality, significant irreversible health effect with reduced life expectancy.

Mechanism of injury Refers to the way the injury occurred, and the forces impacted on the body to cause an injury. These include, falls, slips and trips, object hit on body, moving objectives, exposure to heat, electricity and other environmental factors, sound and pressure, muscular stress, contact with chemicals and other substances, biological exposures and other unspecified mechanisms of injuries.

Medically Treated Injury (MTI) Any injury or illness that results in medical treatment but does not result in any lost or restricted shifts.

Minor Injury (MI) A minor injury is categorised as an injury sustained at work that requires a first aid treatment. Incidents where pain or disability were reported but did not require treatment may also be recorded as a minor injury.

Health and Safety continued

MEASUREMENT BASIS continued

• An injury occurs outside of the workplace;

• Conditions occur at work due to non-work-related tasks.

• When injuries are not reported at the time of incident and the incident is not able to be confirmed as the cause of the injury after investigation.

A non-work-related injury is recorded when:

Non-work-related injury

DEFINITION/BASIS

Key term

Occupational disease Refers to instances where an Occupational Physician diagnoses a worker as having an illness, impairment, or as otherwise having their wellbeing affected due to repeated or long-term exposure to workplace hazards, agents or events, such as silica, diesel particulate matter.

Restricted Work Injury (RWI)

Any injury or illness that results in a person not being able to perform their normal duties and results in:

• A worker being placed on restricted or selected duties because they are not able to carry out all the routine job functions* of their usual role for one or more shift/s; or

• A worker being unable to work a full regular shift i.e. limitation of hours of work

• A work injury that has not resulted in a return to normal duties after 96 hours on a precautionary restricted work classification.

Recordable Injury (RI)

Injuries that result in classification as Medical Treated Injuries (MTIs), Restricted Work injuries (RWIs), Lost Time Injuries (LTIs) and Fatalities.

Precautionary Restricted Work Injury Precautionary restricted duties may be assigned by a licensed health professional (e.g. nurse, physiotherapist, occupational therapist, doctor) after a work-

(PRWI) related injury or illness, where assessment indicates a person is able to conduct normal duties, but restrictions are conservatively applied for any of the following reasons:

• To prevent a more serious strain or sprain. This categorisation is exclusively used for musculoskeletal injuries where the injured person is believed to be fit for work but there is concern that the condition could be aggravated by rough ground, repetitive work, manual handling, vibration etc.

• To reduce likelihood for continued impact and discomfort. This typically applies to preventing the potential for continued bumping or knocking to bruised or otherwise injured areas (e.g. burns, blisters), where the impacts would normally be incidental but due to the location of the injury and work mechanics, could cause aggravation to the injury.

• to facilitate the management of pain or nausea.

• to enable continuous monitoring of a person in a low-risk environment: e.g.: electric shock or injury from electricity at high voltage, whether the person is treated for the shock or injury by or under the supervision of a doctor AND minor electric shocks including periods of precautionary observation or non-invasive monitoring such as an electrocardiogram (ECG).

Total work hours Includes the hours worked for all employees and contractors at all OceanaGold offices and sites within the organizational boundary.