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>