I'll 2nd Nathan and Don.
Round-wound strings are great for O2 hoses, or any other hose/tube of that style. .24ga is good for 72nd, .32ga for 48th, and lookin at around .52 or so for 32nd. (I use "9's" on all my guitars....9, .11, .16 for the highs, and .24, .32, .42 for the wound). Also, old guitar cables are a great source for PLENTY of fine copper wire...much easier to bend, than guitar strings!
I salvage wire from old electronics as well (electrical cords), I have a good stash of various sizes. The stuff I use most, came from the "pig tails" for the old taxiway lighting at my airport. Quite fitting, since 99% of what I build is aircraft!