You've already got obstructive sleep apnea, so we don't have to tell you how important breathing in your sleep is, but this question can very quickly narrow down your mask options.
If you’re a mouth breather because of nasal congestion, allergies, or other difficulty breathing through your nose, you’ll want to try out a full-face CPAP mask first. Or if you would prefer less headgear or a wider field of vision, try an oral CPAP mask.
If you sleep with your mouth open, but don't have issues breathing through your nose, you may want to test the combination of a nasal mask or nasal pillow mask and a chin strap. This can help reduce dry-mouth or sore throat that are occasionally associated with CPAP therapy.
Great news! Nose breathers can use just about any CPAP mask style other than oral masks.
You’ll want to stick with a full-face CPAP mask or a hybrid CPAP mask for the best versatility in airflow.