Similar to OMG (Oh my god!), but ending with a fashionable "osh" to imply "Oh my gosh!", which isn't "Oh my god!", which is important, if importance was the issue. Which it's not.
How really {girly american girls say omg or oh my god/gosh. Add O's in the beginning to make it even more girly, and combine with even more O's in the word "so" to maximise the whole meaning of it.
Ashley: Oooomigosh, did you see Josh in class earlier today?
Brittney: Omigosh yes! He was soooooooooooo hot!
See omigod. Much like oh my gosh, this is a shortened, valleygirl way of saying it. It's also relatively like oh my god, but substituting gosh for god, and deleting a few "spare letters". Can also be shortened to omg, or having a useless "z" added to make omgz.