There is little known definition for this phrase, and it takes a little practise. It's a phrase used for when one of your friends has something in their mouth; i.e water. You need to sound like an old woman when saying; oooh pat!
I did this to my friend that many times whilst he was drinking he was sick.
Will: (pops up in front of him - 'OOOH PAT!'
Harry: (trys as hard as he can not to spit out his drink; then pukes up everywhere)