You're right. Perhaps the course creators have never watched the "Beverly Hillbillies," so you'll need to report it when you get a chance.
There are several informal English words that are equivalent to "oma." I'm sure they vary from region to region. Same with "opa," "ma," "pa," etc.
Where I live, it's common to hear "mamaw" and "papaw" for "grandma" and "grandpa." Most dictionaries don't even list those but in parts of the American South and perhaps elsewhere, those terms are quite common.
So, do report this. And I'll add mine.