Internet slang for awesome. Awe was shortened into a single 'O' and some was changed into sim for a shorter pattern of letters which, when arranged and verballed expounded produces a sound entirely similar to the original word.
Ricky: That's osim mate, good job.
Derek: Yeah, it was pretty awesome, I'm just glad I pulled it off in time.
Ricky: I hear ya, I'd never be able to in that timeframe
Oh Shit It's Monday;
The opposite of TGIF.
See also HSIM