My boyfriend's mom screamed at him because I used the term "Oh my God"..?

If you are in her house you should respect her house rules or she can just ban you from the house. No one can make you change how you talk but they do not have to allow you in their house. It is that simple. Her own swearing is not an issue since it is her house. The bottom line is if you want to go to his house you have to accept that she has all the power.