For simply oxygen masks, the hospital rooms from the private clinics, which are typically comfier than public hospitals', or a sort of hotel room, or even my own home.
I lay down on a double bed. There's an oxygen concentrator on the nightstand with a mask on it, your usual simple mask. I'm not sure why certain are green or clear but either way, I put it on my face, adjust it, and glance in a mirror, watch myself breathe, and I just kind of... let myself wander and do my thing, hearing my own muffled voice. I mess with the regulator until I find a sweet spot of air influx and there goes the fun... but if I'm not alone in that room, I put it on the girl, or she does put it on and look at me flirtatiously, then takes it off, hands it to me, and I do the same... and then I put it back on her, and undress her, and perhaps get undressed myself... a few checks later to see if everything's fine I go down on her as she's wearing it... and she climaxes in a very loud way... but oddly enough I'm the one in control of the machine, always. I'm the one who knows it best and I'm the one to regulate her oxygen flow, but never deprive her of any. I would never let anyone be in control of it either, I'd be too concerned the partner would purposely sabotage the play or try to kill me or something.
For anesthesia, your run of the mill operating room will do. If I'm the patient, I'm not fully naked under my gown, but the doctor is female, and she treats me with the sweetest possible hands. If I'm the doctor, I look at her longingly while I'm doing the procedure, all while reassuring her that it's going to be fine, and that I will experience pleasure for two, that next time, she gets to do it to me. I watch her going to sleep while rubbing the tube on my crotch. Weird one lol.