Zoey telling Lola the reason why Nicole left PCA
Obsessive Male Gender Disorder (OMGD) is a condition in which a girl is "boy-crazy," or addicted to the male gender. It is mentioned in Zoey 101 in the episode Surprise when it is revealed that Nicole was diagnosed with it and had to leave PCA as a result. In the episode, after Zoey and Lola meet up again in their dorm room, Zoey puts a bit of a damper on their reunion when explaining that Nicole isn't coming back (and thus can't be their roommate) because she was diagnosed with OMGD and transferred by her parents to an all-girls school.
While it's never explained if Nicole ever ends up overcoming her OMGD, especially since she's never shown returning to PCA, her leaving PCA ultimately allows Quinn to move in with Zoey and Lola, thus becoming their new roommate.