It's basically "Oh my God!" but a non-blasphemous version ;) [The bible says: it's blasphemous to use the name of God, unless you're praying]

Oh my gosh means you are surprised or shocked.

"Oh my gosh" is an expression of surprise. Gosh is a euphemism for God. This can be abbreviated as OMG in text messages.

"Gosh" is a softened version of "God". We have very creative ways of softening "strong" language and swear words. Another example is "gee" or "geez" for "Jesus". As Alex has explained neatly, you use these exclamations when you are surprised or shocked. "Oh my gosh" can be a positive or negative reaction.

You can say " Oh my God " instead of " Oh my gosh "