Oh shit. 'O' is said in a high pitched voice and 'shi' is said in a high pitched but slightly lower pitch than the 'O' - and is said for an extended period of time, could be indefinite. Meant to signify surprise, something funny or something lame.
"I haven't seen you in forever you piece of shi. O shiiiiiiiiiiiiiiiii"
by Rich Shi March 05, 2008