Coronary artery disease (CAD) is one of the most common cardiac conditions, primarily caused by atherosclerosis. Studies have shown that environmental factors such as lifestyle, diet, genetics, and air pollution contribute to the increasing prevalence of atherosclerosis. Despite advances in non-invasive and medical therapies, coronary artery bypass grafting (CABG) remains a definitive treatment in many cases. CABG is commonly performed using cardiopulmonary bypass (CPB), which increases the risk of additional complications. Among postoperative complications, pulmonary events are the most frequent and are closely associated with increased morbidity and mortality. Postoperative pulmonary complications (PPCs) significantly raise healthcare costs, prolong hospital stays, and worsen patient outcomes. Cardiac surgery increases susceptibility to PPCs through factors such as sternotomy, CPB, myocardial protection via hypothermia, and phrenic nerve injury. These factors impair thoracic compliance, diaphragm movement, and mucociliary function. Mechanical ventilation further contributes through inflammatory responses that may cause barotrauma and volutrauma. The incidence of PPCs varies from 1.96% to 43.7%, depending on patient characteristics, surgical type, and diagnostic criteria. With an aging population and more high-risk patients undergoing surgery, the incidence of PPCs is expected to rise. Reported preoperative risk factors include advanced age, active or recent smoking, BMI \>25, heart failure, prior myocardial infarction, chronic lung disease, type 2 diabetes mellitus, hypertension, and ASA score \>2. Intraoperative factors include prolonged anesthesia, CPB duration \>95 minutes, multiple grafts, and use of the internal mammary artery. Postoperative contributors include prolonged ventilation, nasogastric tube use, and insufficient nursing care. Common PPCs include atelectasis, pneumonia, bronchospasm, pulmonary embolism, pleural effusion, acute respiratory distress syndrome (ARDS), pneumothorax, hemothorax, and transfusion-related acute lung injury (TRALI). Atelectasis is observed in 30-72% of cases on postoperative chest X-rays. TRALI is the leading cause of transfusion-related death and is characterized by hypoxia and bilateral pulmonary infiltrates. Pleural effusions are associated with low BMI, female sex, atrial fibrillation, heart failure, valve surgery, and anticoagulant use. Phrenic nerve injury, often resulting from internal thoracic artery dissection or cold-induced damage during myocardial protection, leads to unilateral diaphragmatic dysfunction. Hospital-acquired infections, particularly pneumonia, are among the most common and costly non-cardiac complications following cardiac surgery. Pneumonia occurs in 2.4%-20% of cases, while ventilator-associated pneumonia affects 35.2% of patients intubated longer than 48 hours. ARDS is the most severe PPC, involving widespread alveolar injury, endothelial disruption, and non-cardiogenic pulmonary edema. Tension pneumothorax may also occur after chest tube removal. Spirometry is a standard preoperative test used to assess lung function, primarily through FEV₁ and FVC values. These measurements help detect obstructive or restrictive defects. FEV₁/FVC \<70% has been identified as an independent risk factor for PPCs. However, spirometry's predictive value is limited. A systematic review by Dankert et al. reported sensitivities ranging from 38% to 84% and specificities from 55% to 99%. Combined data yielded a sensitivity of 62%, specificity of 70%, and an area under the curve (AUC) of 0.74. Pulse oximetry alone cannot reliably indicate PaO₂ above 100 mmHg or provide information on oxygen reserve. The Oxygen Reserve Index (ORI) is a novel, continuous, and non-invasive parameter that evaluates oxygenation status in the mild hyperoxic range (PaO₂ 100-200 mmHg). ORI values range from 0.00 to 1.00 and correlate with changes in PaO₂. ORI \>0.24 corresponds to PaO₂ ≥100 mmHg when SpO₂ exceeds 98%, while ORI \>0.55 may indicate PaO₂ ≥150 mmHg. A declining ORI trend approaching 0.24, despite SpO₂ \>98%, can signal impending hypoxemia. Given the substantial impact of PPCs on postoperative outcomes and the limitations of spirometry in accurately predicting these complications, there is a need for more reliable preoperative assessment tools. The present study aims to compare the diagnostic performance of the Oxygen Reserve Index and spirometry in predicting postoperative respiratory complications in patients undergoing coronary artery bypass surgery. The goal is to identify a more accurate and effective method for preoperative pulmonary risk stratification.