Answer: It's not uncommon. OSA is usually a problem with the upper-airway... and for whatever the reason, this occlusion of the upper-airway causes the sufferer to breathe through the mouth instead of the nose. So, the cause can vary quite a bit between individuals. So, if you're not overweight, here are a few scenarios...If you're occluded only when lying on your back, it tends to be a jaw/tongue problem. The tongue may or may not be enlarged, but something is causing it to flop into the path of the airway... therefore occlusion. The fix for this is usually the dental mouthpiece much like a retainer that holds your lower teeth forward and positions the tongue to a more forward/anterior position.People with deviated nasal-septums tend to compound the problem. The nasal passage is crooked or occluded somehow and forces the individual to breathe through the mouth. The fix would be septoplasty... which requires general anesthesia and about a week of recovery (some folks recover faster).Then there are the allergies... this compounds the problem for thin people with swelling tonsils, clogged nasal passages, etc... Not too many remedies for this. You could try antihistamines like Claritin (Loratidine 10 mg), limited to 1 tablet for every 24 hours.One of the other factors I forgot to mention is the enlarged uvula. Sometimes they get so enlarged that they do drop back and block the upper-airway. There aren't any non-surgical remedies for this. Your options are just the uvula-ectomy and another technique that involves burning/cutting part of the soft palate. (I don't have too much first-hand knowledge of this one, saw it on television).So there's a bunch of causes and possible remedies for you. All of these are causes for OSA that may not be related to being overweight. Now there is also "Central" Sleep Apnea, but you did say OSA specifically... so we won't go there.