[musicman691]

Looking at the op's profile he has a bit of conflicting info on what exact OSX version he has but the profile shows he has a mid-2011 21.5 inch iMac (the smaller version of the machine I have). Those machines originally shipped with OSX 10.6.6; they can use the latest OSX version. So OSX 10.8.5 is very doable and is stable with PT10.3.10.

The sticky point will be getting OSX 10.8.5. Try calling Apple customer support at USA - 1-800-275-2273 (not an Apple Store) and say you need to purchase a copy of an old OSX version. Also don't waste time looking for OSX 10.8.5 on the AppStore as well - you won't find it.