Dynamic magnetic resonance imaging (MRI) allows real-time characterization of upper airway collapse in sleeping subjects with obstructive sleep apnea (OSA). The aim of our study was to use sleep MRI to compare differences in upper airway collapse sites between BMI-matched subjects with mild OSA and severe OSA. This is a prospective, nested case–control study using dynamic sleep MRI to compare 15 severe OSA subjects (AHI >40) and 15 mild OSA (AHI <10) subjects, who were matched for BMI. Upper airway imaging was performed on sleeping subjects in a 3.0 T MRI scanner. Sleep MRI movies were used by blinded reviewers to identify retropalatal (RP), retroglossal (RG), and lateral pharyngeal wall (LPW) airway collapse. Mean AHI in the severe OSA group was 70.3 ± 23 events/h, and in the mild group was 7.8 ± 1 events/h (p < 0.001). All mild and severe OSA subjects demonstrated retropalatal airway collapse. Eighty percent in the mild group showed single-level RP collapse (p < 0.001). All subjects in the severe group showed multi-level collapse: RP + LPW (n = 9), RP + RG + LPW (n = 6). All severe OSA subjects showed LPW collapse, as compared with three subjects in the mild group (p < 0.001). LPW collapse was positively associated with AHI in simple regression analysis (β = 51.8, p < 0.001). In conclusion, severe OSA patients present with more lateral pharyngeal wall collapse as compared to BMI-matched mild OSA patients.