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!'
Kid: "OHSH!"

Parent: "What did you say?"

Kid: "I didn't cuss, if that's what you mean!"
by Risu August 28, 2008
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