It's an ad-lib and a replacement for lyrics you can't think of. It's very commonly used in today's rap generation.
Something similar to skkrt but "ouu" is user alot more.
How to make fun of your homie after his dick went soft before reaching an orgasm during sexual intercourse
Johnny: Bro I was balls deep in Victoria when I suddenly turned into a wet spaghetti noodle! I had to throw my shorts on and run home.
Carlos: HAHAHA YOU WENT OUU SCHLUU! OUUUU SCHLUUU! HAHAHA