This article is from the Anime FAQ, by Steve Pearl with numerous contributions by others.
When the Englis licensors (Studio Proteus & AnimEigo) asked Fujishima what they should call the American release, and he said he didn't care, as long as they felt the title they chose sounded best in English. Therefore, they chose "OH MY GODDESS!", based upon their own opinions. Fujishima never expressed a preference for "OH" and he later told Hitoshi Doi that they should have left it alone. - From a post by Ryan Mathews