I realize how inconvenient it is for you but from what I understand, Form C is mandatory for "foreigners" according to GOI when checking in to hotels.
Reference - http://boi.gov.in/content/form-c
This hotel is perhaps paranoid about security checks (perhaps from past experience) that they are asking it.
Now, I'm not entirely sure if OCI card holder can be termed as "foreigner" though. It probably isn't which is why you were able to check in to other bigger hotels without any issues.
Please let us know how it goes.