It could either be a word or sound expressing pain, a word that is meant to flatter another, or simply a term that is used to embarrass someone else.
However - when it comes down to it, the words is virtually meaningless.
Additionally, it is the 'phrase' that Lil' Wayne says in many of his 'songs'.
Bill - "Hey girl, you look fine today."
Nick - "Ow, get it!"
A guy walks into a wall.
Jim - "Yo momma is so fat, she sweats in December. Imagine what she smells like in May!"
Random guy - "Ow, burn..."
Lil' Wayne - "OW!"
“I hear that O.W.-ing on a regular basis can cure sinus infections.”