"Oh my God!" is typically used an exclamation of surprise or disbelief, as in: "Oh my God! Did you hear what happened to your sister?" "Oh my God! I've just won the lottery!" It can be used in both positive and negative scenarios. "For God's sake" is typically used as an exclamation when someone is angry or upset, as in: "For God's sake! I've told you to clean your room five times already. Why haven't you done it?" (You're having a bad day and yet another bad thing happens to you) "Oh, for God's sake!" Keep in mind, however, that many people may find these phrases impolite or disrespectful, depending on their religious beliefs.

The context or situation where the words are used. Generally for god's sake means you are frustrated with something or someone and don't want that situation happening again. Oh my god means when you are either surprised or shock.

Both expressions are very similar but in my experience "oh my god" is used more generally to show disbelief, frustration, excitement or other outbursts of emotions. "For god's sake", in my opinion, and I could be wrong, is used more like a curse to express negative emotions like frustration. So if your coworker Terry accidentally spills coffee over your files you could say "for god's sake, Terry!" but later when you retell the story you could start with "oh my god, you won't believe what Terry did today!" I hope this was correct and helpful :)

For god's sake is when and where all other reasons and arguments fail, we tend to turn to God .... and plead "for God's sake" ..... as if that's the ultimate excuse that we may put forward to convince a person or justify an action or whatever..... But actually as i searched in google about that is a quotes in a 2002 film ! And OMG you definitely know what bros !