Background: Obstructive sleep apnea (OSA) is a known risk factor for postoperative atrial fibrillation (POAF) after cardiac surgery. However, whether better management of OSA reduces the risk of POAF remains unknown. The aim of this study was to determine if postoperative positive airway pressure (PAP) treatment for OSA reduces POAF risk after cardiac surgery. PAP included both continuous and bilevel positive airway pressure. Methods: This retrospective cohort study was conducted at Stanford University teaching hospital. We included a total of 152 OSA patients with preoperative electrocardiography showing sinus rhythm who underwent coronary artery bypass grafting (CABG), aortic valve replacement, mitral valve repair/replacement, or combined valve and CABG surgery from October 2007 to September 2014. Postoperative PAP use status was determined by reviewing electronic health records. The primary outcome was time to incident POAF. We reviewed records from the time of surgery to hospital discharge. Multivariate Cox regression model was used to calculate the adjusted hazard ratio of postoperative PAP in association with risk of POAF. Results: Of the 152 OSA patients included for analysis, 86 (57%) developed POAF, and 76 (50%) received postoperative PAP treatment. POAF occurred in 37 (49%) of the patients receiving postoperative PAP, compared with 49 (65%) of those not receiving postoperative PAP (unadjusted p value = 0.33). Multivariable Cox regression analysis of time to incident POAF did not show an association between postoperative PAP treatment and risk of POAF (adjusted hazard ratio: 0.93 [95%CI: 0.58 – 1.48]). There were no significant differences in other postoperative complications between the two groups. Conclusions: The study did not find an association between postoperative PAP treatment and risk of POAF after cardiac surgery in patients with OSA. Future prospective randomized trials are needed to investigate this issue further.