It's 'oh, shit!' fast, and cut off into 'oh, sh--!' because sometimes someone you can't swear around is in your vicinity. It becomes 'ohsh', following perfectly from the beloved 'oh, shit!'
Simply "asshole" spelled backwards. It's meant to be funny cause it sounds spanish but anyone who speaks spanish will have no idea what your talking about.
Last night I was chillin' at the club and some El Ohssa stepped on my Pumas.