You may chose to disagree on this but at least consider it!
God has had a problem with the misuse of His Holy name for a long time. Let's go back at least to the book of Exodus, and the 3rd commandment, "You shall not take the name of the Lord your God in vain, for the Lord will not hold him guiltless who takes his name in vain.”
Now some will argue that this means not to use the Lord's name as a curse word. I'd agree, but not stop there because any misuse of God's name is misuse of God's name, period!
Using the Lord's name in vain simply means it is not held in high esteem, not given special attention or honor, and not considered sacred or set apart, but used in a common way. The Psalmist said God's name was (and is) majestic, holy and awesome.
So when I hear people say "oh my God" 15 times in describing something, how much more common can it be? It is so commonly used this way that most people are somewhat unaware of just how much they say it. It's not the words 'oh my God' but the careless, casual and slang use of it. There could be a time where you could call out to God in a situation using the same words in an appeal for His mercy or attention and it be acceptable.
But using OMG at the beginning of every sentence, certainly makes it common place and isn't an appeal to Him at all. There are TV shows that use it, even in the title of the show. There are hot keys on apps, and even the fact that we have reduced it to the "OMG" acronym proves my point! It's on Facebook so much it's likely one of the most frequently used phrases we see. More proof.
Many will say they mean no disrespect by it, but yet they use the phrase as though it were an exclamation point in every conversation. It does carry meaning and it is disrespectful toward God if for no other reason than He commanded us not to use it in a casual flippant way, and certainly God had more reasons than that, perhaps one being that He's God, and nobody else is, and that a pretty big deal!
If we can't discipline ourselves to at least hold His name sacred, how much attention will we pay to His commands, or His Son?