Which camera?
With the Mak (like the SCT's) there shouldn't be a problem with back focus.
There are many "good" OAG's available - BrightStar/ Orion, Celestron Radial, and I like the new Telescope Service -siding mirror arrangement (Not the narrow 9mm OAG - tooooo many issues!!)
http://www.teleskop-express.de/shop/...rt-design.html