when you drop your cell phone in the rode and your about to get ran over by a car and you really say o shit but the way you say it makes your friend think you said ooosh.
"Ooossshh" is a word made popular by Keith Lemon, but in fact the word has been around since time immemorial. It describes the effect of great excitement and is the sound emitted when pressure is released when opening a can of shaken lager. It can also be the description of other activities.
I was rigourously sucking the nipples of my heavily pregnantgirlfriend when she ooossshhhed all over my moustache.