The frequency of obstructive sleep apnea (OSA) among patients with atrial fibrillation (AF) has been previously studied; however, only few studies have investigated the frequency of AF in sleep disorders, with growing evidence suggesting that OSA is considered a risk factor for AF. Aim The aim was to assess the frequency of AF among patients with OSA and to define the predictors of AF in such population. Patients and methods This cross-sectional study enrolled patients with OSA diagnosed by a full-night polysomnographic sleep study. AF was diagnosed through patients’ history and standard 12-lead resting ECG. Results A total of 374 patients with OSA were enrolled, with a mean age of 50.1±12.2 years. Male sex represented 54.5% and 14.79% were smokers. Overall, 19.25% presented with mild degree of OSA, 18.98% with moderate degree, and 61.7% with severe OSA. Most patients with AF were smokers (53.6%, P<0.001), and 46 of 58 patients had severe OSA, P=0.004. Overall, 59.3% of those with ischemic heart disease (IHD) and 59.1% of those with congestive heart failure had AF, with a highly significance difference (P<0.001). Incidence of AF showed significant increase with increased OSA severity. Multivariate logistic analysis revealed three independent predictors of AF in patients with OSA: severe OSA [Odd ratio (OR): 3.2, P=0.003], IHD (OR: 10.06, P<0.001), and current smoking (OR:9.2, P<0.001). Conclusion AF was evident in 15.5% in patients with OSA. The incidence of AF was higher whenever the OSA severity increased. Three independent predictors of AF in patients with OSA concluded were severe grade of OSA, IHD, and current smoking.