We demonstrate a new imaging test that uses simultaneous multi-slice real-time MRI during continuous positive airway pressure (CPAP) to quantify upper airway neuromuscular reflex and passive collapsibility. Both are measured using cross-sectional area fluctuation during abrupt changes to CPAP. We applied this test to patients with obstructive sleep apnea (OSA) and healthy controls. Subjects with OSA showed 3-5x higher airway area fluctuation compared to healthy volunteers, and this difference was statistically significant (p < 0.05). Neuromuscular reflex and area fluctuation varied greatly among the OSA patients, suggesting a potential spectrum of active/physiological and passive/anatomical factors contributing to OSA.