The hardest thing about learning anything OO related is simply getting over the language barrier. Anytime you talk to an OO afficionado, you always end up hearing a term you don't understand. My biggest problem with learning Model-Glue, Reactor and ColdSpring, three things I really want to start using, is finding out what IOC, AOP, DAO, Beans, Transfer Objects, Gateway Objects, and all of the other terms mean, so that I can actually make it all the way through the documentation. Google searches are alright, but I've never had one spot I could go to find out what it all meant.
To fix that, I put a copy of CanvasWiki, by Ray Camden up at ooGlossary.com. If you have a good definition for a term, please feel free to add it. These will hopefully be edited by the community at large, and we should end up with a good resource for all of that OO language goodness.