The noise a person might make when they are close to shitting themselves. Often made during the run-up to the toilet to signal to others they are desperate. It is like an extended "oh no", but very drawn-out to add drama and excitement to the situation. This noise may be heard throughout the shitting process, and will often be accompanied by vigerous sweating and closed eyes due to severe pain yet a sense of relief.
(Running to the toilet), ooomph! oooooomph! (People move out of the way). (Sitting on the toilet, with closed eyes). Ooomph! Ooomph! "Dude, can you hear that guy shitting? He must be desperate". Oooooooooooomph. (Leaves toilet).