Opp. is a written short form of opposite. You find it in dictionaries, word lists, and textbooks, sitting after a word to point you to the word on the other side of the meaning: hot, opp. cold. It can also mark the thing that sits across from something, as on a plan. People do not really say “opp” out loud as a word; when they speak, they just say “opposite.” In messy notes a student might use opp. for other words that start the same way, but in a published learner’s dictionary this one means opposite.