The mainstream dictionary entries seem to be either for "opp" meaning "opportunity" (as in "business opp", "job opp"), or as an abbreviation for terms such as "opposite" or "opus". So I think the hip-hop slang is probably a separate coinage.
Distribution of slang term “opp” - English Language & Usage Stack ...
single word requests - What's the opposite of "precede"? - English ...
Neither shallow nor gentle are the opposite of steep, but rather words we make do with because there is no actual word for what we mean; when asked the opposite of gentle or shallow we wouldn't naturally say steep, but harsh or deep respectively. Is there a general name for the situation where words 'should' exist but don't, and we have to make do with others?
single word requests - What is the opposite of "steep"? - English ...
Opp. to rare. and 'rare' in this sense: a. Originally: (of an organ or tissue, soil, or other substance) having the constituent material or particles loose or not closely packed together; not dense or compact; attenuated. In later use chiefly: (of air or a gas) having low density, thin (though cf. rare gas n. at Special uses 2).
Possible Duplicate: How do I say “Our meeting is preponed”? A friend of mine asked me this question, and it caught my curiosity. Is there an explicit opposite of the word postpone?...
verbs - What is the opposite of postpone? - English Language & Usage ...
API reference for the CObArray MFC class which stores CObject pointers in an array.
Seeking Alpha: OPP May Expose Investors To Illiquid Risk During A Time Of Heightened Economic Uncertainty