Aim: While arterial blood gas (ABG) analysis is invasive, intermittent, and costly, the oxygen reserve index (ORI) is a new method that can be non-invasive and continuous measurement aimed at providing information about the patient's O-2 status in the moderately hyperoxic range. In our study, the ORI to PaO2 relationship in different fresh gas flows was evaluated.,Methods: This randomized prospective study was conducted between November 2018 and November 2019. All patients were ventilated for the first 10 min after intubation with 50% O-2/air and 6 L/min fresh gas flow. Then, the flow rate was randomly set to 4 L/min for high-flow anesthesia (group H) or 1 L/min for low-flow anesthesia (group L). ABG's were taken before preoxygenation, intraoperative 60th min, and at the end of surgery, and simultaneous ORI and SpO(2) were recorded.,Results: The study was completed with 70 patients. Mean PaO2 values were higher in group H, apart from before preoxygenation (p<0.05). Mean ORI values differed between groups except before preoxygenation and the intraoperative 10th min (p<0.05). A statistically significant, positive and weak correlation was identified between ORI and PaO2. According to the regression analysis, the ORI value was approximately 0.2 when the PaO2 value was >= 100 mmHg at the intraoperative 60th min and at the end of the surgery, and 0.3 when the PaO2 was >= 150 mmHg.,Conclusion: ORI may be an alternative to PaO2 in monitoring the oxygen status of intraoperative patients.