Kinda like saying "Oh lord". Obviously, "Oh" = "o", and "Lord" = "LAWD". Saying LAWD instead of LORD is like saying LAWL instead of LOL.
<n00b> I HAX U
<me> O LAWD! WHAT AM I GONA DO NOW?
<fag> So, how's the weather.
<me> o lawd, it's a hurricane! I think im loosing interweb!
<fag has now been blocked>
A variation of "Oh Lord!". The spelling is based, phonetically, on a stereotypical southern African-American pronunciation. Rather like "O ma gawd".
Guy1: And then I totally hit dat shit.
Guy2: o lawd.
Grandpa: Back in my day, we used pine cones and tampons!
Impressionable Children: o lawd, here we go again.
KFC Clerk: May I take your ord-
Black Women: O LAWD GET ME A BUCKET O' CHICKEN.