Introduction: Obstructive sleep apnea (OSA) is characterized by recurrent airway obstructions, resulting in decreased oxygen levels and disrupted sleep. These episodes can manifest as hypopnea or apnea, affecting respiratory muscle function, lung capacity, and chest compliance. OSA significantly impacts exercise tolerance and aerobic capacity, with severity influencing these limitations. Aim: This study aims to evaluate lung function, respiratory muscle strength (RMS), and functional exercise capacity (FEC) and to investigate how various factors affect respiratory parameters and FEC in individuals with OSA. Methods: The study included 30 patients aged 18–80 years, of both genders, diagnosed with OSA and chronic obstructive pulmonary disease. Those with significant cardiopulmonary, neuromuscular, or orthopedic disorders affecting functional capacity were excluded. Lung function was measured using a Medical International Research (MIR) Spirometer for forced vital capacity (FVC), forced expiratory volume in one second (FEV1), and the FEV1/FVC ratio, while RMS was assessed with a manovacuometer for maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP). FEC was determined using the six-minute walk test. Results: The mean values for FVC, FEV1, and the FEV1/FVC ratio were 1.44 (0.51), 1.29 (0.42), and 92.87 (7.08), respectively. The mean distance covered in the six-minute walk test was 92.35 (52.53). There was a positive correlation in RMS and a positive relationship between lung function and FEC in patients with OSA. Conclusion: The findings indicate a decline in lung function, RMS, and functional capacity in OSA patients.