Context <p>Obstructive sleep apnea (OSA) is highly prevalent in patients with obesity and contributes to metabolic dysfunction and cardiovascular burden. However, its association with cardiopulmonary fitness in metabolic bariatric surgery candidates remains less well characterized.</p> Objective <p>To examine the association between OSA severity and preoperative cardiopulmonary fitness in patients undergoing sleeve gastrectomy (SG).</p> Methods <p>A retrospective cohort study including 104 patients who underwent SG with preoperative sleep evaluations and Cardiopulmonary exercise testing (CPET). Data from clinical characteristics, portable sleep monitoring studies, cardiopulmonary fitness, and metabolic measurements were collected.</p> Setting <p>A university-affiliated hospital.</p> Results <p>Higher OSA severity was associated with lower oxygen uptake (VO₂) at anaerobic threshold (AT), reduced power output, blunted heart rate (HR) responses, and decreased oxygen uptake efficiency slope (OUES). After adjustment, higher OSA severity remained significantly associated with lower PowerAT, HRAT, and OUES, among other CPET indices. This effect was observed in both male and female subgroups.</p> Conclusions <p>Higher severity of OSA was associated with reduced preoperative cardiopulmonary fitness in patients undergoing SG. These results may underscore the potential importance of screening for OSA as part of the comprehensive preoperative evaluation in this population.</p>