The primary objective of this study is to evaluate the effectiveness (prevention of thromboembolic events) and safety (major bleeding) of adding oral anticoagulation (OAC) to background antiplatelet therapy in patients who develop new-onset post-operative atrial fibrillation (POAF) after isolated coronary artery bypass graft (CABG) surgery. All patients with a qualifying POAF event, who decline randomization, will be offered the option of enrollment in a parallel registry that captures their baseline risk profile and their treatment strategy in terms of anticoagulants or antiplatelets received. These patients will also be asked to fill out a brief decliner survey.