Oy, am I thoisty!
For those of you interested in things Yiddish, there's a most enjoyable 28 minutes of listening on NPR - Terry Gross in conversation with Michael Wex, author of Born to Kvetch: Yiddish Language and Culture in All of Its Moods.
'Oy' basically means... 'Oy'. If you've got to ask, you're fortunate enough that you'll never know.(Thanks: L.)