I think the complaint of something being "OOC" is often a bit unfair. People in real life aren't actually particularly consistent, they often do things you don't expect them to do, and something as simple as being tired or having a bad day can change the way they act to an extent. Nobody really has a 100% consistent personality, the you today might be a bit different from the "You" of tomorrow. So, I don't like to jump on screaming "OOC" every time a character does something a bit out of the ordinary, instead I examine my image of the character and try to think either of how this out of the ordinary moment can fit in with the rest of what I understand of them, or I allow my understanding of them to change and grow based on the new information.
That said, there's a limit to that. People are a little different each day, but that only goes so far and if there's a drastic change in behavior there's usually a reason for that. You couldn't just have say...Rainbow Dash suddenly quit the Wonderbolts for no reason. Because that makes no sense what-so-ever and goes against every single thing we know about the character. If you want to make a drastic change to a character you can do it, but you should think of good explanations and reasoning. So you could write RD quitting the bolts and have it make sense and be a good move for the character, but you need to provide a good reason for it, because a drastic change with no explanation isn't really good writing. I'm willing to accept a lot from characters provided there's some good thought put into them.