Abbreviation for the term, 'Oh Our God,' used rather than, 'Oh My God.' Some people (mostly super religious ones) are offended that you would think that God "belongs" to you, so now if you ever meet a crazy religious person, make sure you make it seem as if you "share" God.
It is similar to WTF without having to use the (overused) acronym. It's more dominating because it is steaming with frustration and annoyance, not just disbelief. The new-age version of ugh.
Kenny G says "Ugh, my hair is caught in the car door again." while Amy Winehouse says "Oog, I think that was chalk!"
It is especially helpful to those that work in an office setting that contains a lot of Mormons, less cursing with more feeling.