O.I.F. means “Oh, if I find out!”, referring to someone discovering their partner is cheating. Created by the rap group Wreckx-n-Effect, it’s a play off of O.P.P.
This was a funny lyric in one of their songs. Wasn’t meant to rival O.P.P. by Naughty by Nature. It was funny though! Don’t remember anybody in the hood or the suburbs saying it.