Aim 2 seeks to identify subgroups of patients that have the greatest reduction in AHI (responders) with each treatment (post-hoc). We will use baseline physiological measures of the four traits causing OSA (collapsibility, responsiveness, loop gain, arousal threshold) to determine which characteristics predict responses to each intervention (leave-one-out support vector machine modeling). Clinical measures of the same traits will be estimated from the placebo night to confirm that responses can be predicted with clinically-available data. We will also test whether responders to oral appliances have a greater response to oxygen than oral appliance non-responders (and vice-versa), to address whether responders to both treatments are similar or different.