Introduction <p>The oxygen reserve index (ORi™), a non-invasive variable that continuously reflects oxygenation, was first reported in 2016. With the 2018 update of ORi, the scaling between 0.00 and 1.00 was modified. This article provides a follow-up report on the relationship between the updated ORi and arterial partial pressure of oxygen (PaO₂), based on our previous study using the original version of ORi.</p> Methods <p>The updated ORi version analyzed in the present study used a Revision M sensor. Twenty adult patients who were scheduled for surgery under general anesthesia with arterial catheterization were enrolled. After induction of general anesthesia, arterial blood gas analysis was performed with the fraction of inspiratory oxygen (FiO₂) set at 0.33. The PaO₂ and ORi at the time of blood collection were recorded. After that, FiO₂ was changed to achieve an ORi of around 0.5, 0.2, or 0, followed by arterial blood gas analysis. The relationship between ORi and PaO₂ was then investigated using the data obtained.</p> Results <p>Seventy-six datasets from the 20 patients were analyzed. When PaO₂ was &lt; 240 mmHg (<i>n</i> = 73), linear regression analysis showed a relatively positive correlation (<i>r</i>² = 0.4683). The cut-off ORi value obtained from the receiver operating characteristic curve to detect PaO₂ ≥ 150 mmHg was 0.45 (sensitivity 0.833, specificity 0.810). Four-quadrant plot analysis demonstrated that ORi has good trending ability with respect to PaO₂ (concordance rate was 100.0%).</p> Conclusion <p>Although the original and updated versions of ORi demonstrate similar properties regarding their ability to track PaO₂ changes, the updated version has a wider absolute value range. Therefore, caution is warranted when interpreting ORi values, as absolute values may vary significantly between versions, even at the same PaO₂ level.</p>