Oma Opa vs. Großmutter Großvater
Is there a certain context to use Oma and Opa or does it not matter?
To me they sound a combination of old-school and childish. I'm a native speaker and never heard anyone say Großvati and Großmutti, I only read it. It's Oma and Opa in informal context, sometimes Omi or Opi but that mostly to the grandparents themselves (it's like the difference between dad and daddy, but for grandparents).
Children have all sorts of nicknames for their grandparents -- different families often have different traditions.
So some might say Oma, some might say Omama, some might Omi, some Großmutti, etc. etc.
A bit like in English - some say nan, some nana, some granny, some grandma, etc.