I lived among Koreans for several years. A phrase that sounds like that was splashed into many sentences.
Once I asked my friend (who frequently used it when talking to her children on the phone) what it meant.
She told me “Mother says” is the definition.
A year or so later I was talking to a young Korean attorney and she said that most of the phrases and words I remembered (I was telling her the words I know) are “filler words” and of course things you see on a menu or ask a food server.
On direct discussion of the sound OH MAH GAH she told me something quickly and I stated that my friend said it means “Mother says” to which she replied something like "no, it means {something I did not memorize so can not repeat here and can only describe as not mother says} then she said something like um well yes it can mean that but usually we say it to mean {}.
So today I came looking on the Internet to see that other meaning but can not find it.
I guess I have to be content that when a mother is talking to her children it may mean Mother says and just let it go at that.
There are other sounds that just get put into sentences so they flow and the ever popular jiguum which means “right now” and is used quite frequently to round out a sentence as far as I know and that was another of the filler words according to the young Korean attorney.
I would love to have a complete understanding.