OCI/PIO Application based on deceased parent/spouse

My father was an Indian national so I would like to apply for an OCI and similarly my mother would like to apply for a PIO (as he was her spouse).

He died in 2009 but his Indian passport expired in 2003.

He was born in the 1920's in India and as far as we are aware he did not have a birth certificate so the only documentary evidence we have is an expired Indian passport.

My grandparents were also Indian (his parents) but again, I have no evidence to prove this.

Does anyone know where we stand with regards to getting OCI/PIO cards?

Has anyone else had a similar experience of applying for OCI/PIO based on a deceased relative?

Any help is gratefully appreciated.

I don;t have first hand exp. But you can apply for PIO card if your father last passport was indian(i.e if he did not acquire any other country citizenship or your mother )