Just purchased an OS .91 FX and it is ringed (my first)...What is the best way to break it in?

Typically any engine that is ringed needs to be broke-in slowly and VERY rich...so much that you almost need to leave the glow ignitor on the first coupla tanks. After that slowly lean it out, little by little, so on the 13-15th tankful you are at full lean + a few clicks open. Any different than this would probably work just fine...depends on how long you want the engine to last though. You can either do it right or cut a few corners...the choice is yours.

The November 2001 issue of Model Airplane News has a very good article on engine break-in by Dave Gierke.

I'm with Gene here...run it very rich, slobbering rich for the first 5 or 10 minutes, then slowly lean it over several runs until it's just below a 2 stroke for maybe a total of an hour. After 30 minutes or so I like to vary the throttle so the load changes. One thing that has to be realised is that it's not just the fit against the liner that's being determined but it's also the fit of the ring against the bottom of the ring groove. This must be perfect to hold a proper gas seal. I prefer to use at least 20% castor oil for running in as well.
It may seem like a lot of work compared to an ABC but it's well worth it. A ringed engine can last much longer than an ABC and usually only needs a comparatively cheap ring fitted to restore it to virtually as new.

Thanks for the good info...I can't wait untill the snow goes away..

Don't forget that you should start out with a fuel which contains a little castor until that ring seats. Omega 15% would be ideal and afterwards you can switch to Coolpower or another synthetic.