Some common nicknames for grandmothers are grandma, granny, nana and granny. Mimi, gram, nanny oma, mamaw and gran are also common nicknames that kids use to refer to their grandmothers.
Choosing what to call a child's grandparents is one of the decisions that new parents have to make. Parents can either choose which names the child should use, or they can let either the grandparents or the child come up with the nickname. Another alternative is to use the nicknames that have been used to refer to grandmothers in the family for generations, whether that is oma or the more common grandma.