is it possible to override a function in base class which has already overridden a function from inherited contract of openzepplin but doesn't have virtual keyword in base class while overriding it.

can you explain a bit better?

override(override1, override2, override3, etc.)

Say there is a erc20 contract that has transfer function which over rides transfer function present in openzepplin erc20 library.

Now if I upgrade the contract with inheritance of original contract, and want to change behaviour of tranfer function, will it be possible to over ride the transfer function in original contract in new inherited contract

yes

override(openzeppelinERC20, OldContract)

on the new one

Thing to notice here is Old contract doesn't specify virtual keyword

thats ok

This isn't true. Unable to get this working. It keeps complaining that OldContract needs to have virtual keyword to override it's function in child contract