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1.
an exclamation most commonly used if one if horny and/or has just witnessed a very horny situation, such as fucking.
Alex: Yo, im going over to Jan's hosue tonight to have some fun

Steffi: Oh baby!
by oh, baby, horny, fucking, fuck November 24, 2008
 
2.
When a hot girl goes by or a hot car, something sexy or beautiful, you holler "oh baby"
"Look at that hot girl man, shes walking by right now, oh baby!!!"
by Alan Pugach Barker December 06, 2006