After obtaining ethical committee approval, this single-center, prospective, observational study included volunteer patients of both sexes, aged 65 years and older, with ASA II-III risk scores, who were scheduled for elective surgery with endotracheal intubation under general anesthesia (GAA). 90 patients were divided into two groups: those with normal BMI (19 \< BMI \< 25 kg/m² (GROUP 1)) and those who were obese (30 \< BMI \< 40 kg/m² (GROUP 2)). In addition to standard ASA monitoring, a finger sensor (Rainbow® SET, Masimo Inc., Irvine, Ca., USA) was implanted for ORi measurement, and baseline values were recorded. Patients were preoxygenated until EtO2 (endtidal oxygen) concentration reached at least 85%. After ORi reached a plateau level, general anesthesia was induced and endotracheal intubation was performed. After confirming the tube's position, the breathing circuit was disconnected from the endotracheal tube, and patients were allowed to be apneic until SpO2 reached 94%. Throughout this process, patients' other vital signs, SpO₂, and ORi were continuously monitored. The time elapsed when ORi triggered an alarm, when SpO2 reached 97%, and when SpO2 reached 94% was recorded. The 'ORi warning time', 'SpO2 warning time', and the 'additional warning time provided by ORi' (the difference between 'ORi warning time' and 'SpO2 warning time') were recorded.