The purpose of this study is to evaluate the effects of treating Obstructive Sleep Apnea (OSA) with continuous positive airway pressure (CPAP) therapy on symptoms of post traumatic stress disorder (PTSD). The study hypotheses are that CPAP use will improve PTSD symptoms overall and that CPAP use will improve sleep quality and duration, nocturnal symptoms related to PTSD, mood, daytime sleepiness, sleep-related quality of life, and general health perception.