Background: Obstructive sleep apnea (OSA) is a common sleeprelated breathing disorder associated with excessive daytime sleepiness, cardiovascular morbidity, metabolic dysfunction, and impaired quality of life. Continuous Positive Airway Pressure (CPAP) is the standard treatment for moderate-to-severe OSA, but real-world effectiveness depends on adherence and impacts beyond apnea reduction. Aim: To evaluate the effectiveness of CPAP therapy versus standard care in improving respiratory indices, daytime sleepiness, blood pressure, and quality of life in adults with moderate-to-severe OSA. Methods: In a single-center, randomized controlled trial conducted from January–December 2019, 120 patients with polysomnographyconfirmed moderate-to-severe OSA (Apnea-Hypopnea Index, AHI ≥15 events/hr) were randomized 1:1 to CPAP treatment (auto-CPAP with education and adherence support) or control (sleep hygiene advice and lifestyle counseling). Primary outcome: change in AHI at 3 months. Secondary outcomes: Epworth Sleepiness Scale (ESS), daytime systolic/diastolic blood pressure (BP), and SF-36 quality-oflife score. Intention-to-treat analysis performed. Results: At 3 months (CPAP n=58, Control n=57; 5 lost to follow-up), mean AHI fell from 34.2 ± 9.8 to 6.4 ± 4.1 events/hr in the CPAP group (mean change −27.8; 95% CI −30.9 to −24.7), versus 33.6 ± 10.1 to 29.1 ± 9.6 in controls (mean change −4.5; 95% CI −6.7 to −2.3) — between-group difference p < 0.001. ESS decreased by 6.2 ± 2.8 points with CPAP vs 1.1 ± 2.3 with control (p < 0.001). Mean systolic BP decreased by 6.8 ± 9.2 mmHg with CPAP vs 0.9 ± 6.5 mmHg in controls (p = 0.002). CPAP users reported significant improvement in SF-36 physical and vitality domains (p < 0.01). Average nightly CPAP use was 5.1 ± 1.4 hours; higher adherence (>4 hours/night) correlated with larger improvements. 

Conclusion: CPAP therapy produces large, clinically meaningful reductions in AHI and daytime sleepiness and yields modest but significant reductions in blood pressure and improvements in quality of life at 3 months. Adherence strongly influences effectiveness; programs to improve CPAP use will maximize clinical benefit.