OCI on father or grandfather?

Hello everyone,

So my situation is as follows:

I was born in the UK, my mother is British by birth as are my maternal grandparents.

My father was born in India is in 1959. He came to UK in 1985 and married my mum and thus gained British Citizenship.

My parents are now divorced and I don't have anything to do with my father. So asking him for paperwork is going to be difficult.

My paternal grandparents live in India and hold indian passports. I have an excellent relationship with them and they are willing to help me. So in this case, is it sufficient to have:
1. my birth certificate (showing my father)
2. my Dad's birth certificate (showing his parents.)
3. My grandfather's indian passport/ grandmother's Indian passport

I have no contact with my dad so asking him for his old passport, school certs etc would be difficult. What else do I require? Also, I have been told by a family member that that my dad has OCI in his passport already.

What would be the easiest for me to do? Through grandparents? Or is my case not strong enough through them?

Any help would be most appreciated!

Thank you!