How do you say "Oh My God" in spanish?
"¡Ay Dios Santo Bendito y la Virgen y la Santísima Trinidad!"
Don't worry..It will come naturally. When your car gets stuck on a railroad track and you see the light of a 40 ton engine 20 feet in front of you..You will be surprised at how fluent you become in saying "oh my God"
I think that it is probably a good idea to decide whether you are more interested in a literal translation or in knowing what expressions are commonly used to convey a specific meaning.
That is, the expression, "Oh my God!" is an interjection that can be used to convey certain emotional responses to a given situation. When it comes to the actual words a person prefers to use to convey this emotion, it is generally a matter of style and not substance.
For instance, to express surprise or astonishment over something, there is really little semantic difference in exclaiming any of the following: (Oh) my God!; Oh my dear!; (Oh) my word!; Oh my goodness!; Oh!; wow!; whoa!; Hah!; Dear God!; Dear heavens!; Dear Lord!; God bless!; Bless my stars!; Bless my heart!; Gemony!; Blimey!; What in the world!; or What!
What is important about the phrases above, then, is not so much the words used but the sentiment expressed. In the same way, there are several different expressions that could be used in Spanish as well, depending on the emotional connotation such an expression is meant to convey.
Here are a couple of links, if you are interested, which list (by no means, exhaustively) some of the more common interjections that are used to convey various emotional reactions.