If you avoid trying too hard... I think that'll eliminate one of the major causes of OOC-ness.
That's. So. True.

That also might be why humor fics are usually rejected beucase of OOCness. It's pretty easy to try too hard with humor. Anyways...

With minor characters, I find that they're also easily made OOC. Even if we don't know much about them, I find that when I read a fic that's not written very well, the characters just seem a bit OOC to me. But that's probably just me..

Forcing characters into a situation that wasn't designed for them results in OOC, whereas crafting the story around the characters will be a lot smoother for everyone - characters, readers, and the writer.
That's also so true! There are so many ways a character can act, but compared to the different plot turns and ideas, the amount is vastly lessened. Like, a plot is more flexible than the characters, so characters should be what you revolve (some of) your plot around.

Yet, I think it depends a bit on genre. I'm more likely to forgive an author for making a character OOC (a bit) in humor, and sometimes some general fics. But, I'd prefer real IC characters for dark/angsty and romance fics.

I'm always going on about genres...