"Zij is mijn ma en niet mijn oma!"
Translation:She is my mom and not my grandma!
I don't think any of the informal versions of "grandmother" are universally standard. Gran, granny, grandma, nan etc are all as good as each other, though their use may differ regionally. Granny and gran were what I used, and to me grandma sounds rather old fashioned.
I find that the /o/ sound is a kind of universal for human to describe things that are big or sublime. (something above our level)
Say, in Japanese big is 大きい which reads o-ki-i with an /o/ in English and many others there are "awe" and "awe"-some Now in Dutch, I know that there is o-ma.
This word in quite beautiful and I suppose it to be a quite primitive word with an anthropological origin.
Last but not least ... the sentence its self is a very sad one... for the mother :)