As for its reference to the God of Judaism and Christianity, nothing could be further from the truth.
Christianity, Judaism also Islam, Buddhism, Scientology to mention some are the subject of prophets, religions and cults: each in their own way referring to their perception of God.
For a person to exclaim “Oh my God” they are making a statement between their thought and who or what they perceive as God.
As long as they have that belief in God, which is their birthright, I cannot see how ‘Oh my God’ interferes with the prophetic, religious and cult activities of others.
I won’t desist from using the term and it is not blasphemous.