A foreign national, - (i) who was a citizen of India at the time of, or at any time after 26th January, 1950; or (ii) who was eligible to become a citizen of India on 26th January, 1950; or (iii) who belonged to a territory that became part of India after 15th August, 1947; or (iv) who is a child or a grandchild or a great grandchild of such a citizen; or (v) who is a minor child of such persons mentioned above; or (vi) who is a minor child and whose both parents are citizens of India or one of the parents is a citizen of India - is eligible for registration as OCI cardholder. Besides, spouse of foreign origin of a citizen of India or spouse of foreign origin of an Overseas Citizen of India Cardholder and whose marriage has been registered and subsisted for a continuous period of not less than two years immediately preceding the presentation of the application is also eligible for registration as OCI cardholder. However, no person, who or either of whose parents or grandparents or great grandparents is or had been a citizen of Pakistan, Bangladesh or such other country as the Central Government may, by notification in the Official Gazette, specify, shall be eligible for registration as an Overseas Citizen of India Cardholder.
India became republic on 26th January 1950. That is the day on which they celebrate Republic Day. Your father left in 1949 !!! Your grandparents also left India before 26th January 1950 ??? If your grandparents were citizen of India on 26th January 1950 and if you can prove that with documentation, you have a chance. Your father must be having British Indian passport and later on opted for British nationality.