A disease that afflicts girls who acquire a boyfriend and ignore every other person in their lives except for their significant other. They spend every moment possible with this person, much to the annoyance of their friends.
A girl with OMG!Boyfriend usually doesn't notice she has the disease until said boyfriend breaks up with her two years later and she's left alone and friendless.
I was supposed to go to the mall with Sarah, but she bailed on me to hang out with Steve. She has a horrible case of OMG!Boyfriend Syndrome.