Did you not ask the vendor about the OC, if so what was his reply?
If the building is more than 15 years old, you cannot get an OC for that at this stage.
The completion certificate shall be sufficient.
Actually all those responsible for constructing illegal buildings often get away scot-free and the end user has to pay the price
“It is reckoned that at least 6,000 buildings across Mumbai are paying double money for municipal water, which means they are not authorised. Many unauthorized structures are routinely regularised on payment of penalties... but there are many more that are not yet regularised for various reasons.
there are thousands of apartments in cities like Bengaluru, Mumbai or Delhi which have been occupied without any Occupancy Certificates. In many of the cases, the building plans of these apartments are approved but they do not get OCs due to not sticking to the approved building plan or due to other violations.
Upon the completion of the building construction, the builder or the developer of the apartment has to apply for the ‘building completion certificate’ with the local authorities.
If the building construction is completed as per the building approval plan and if it also meets other building standards like distance from road, height of the building, if rain water harvesting system is in place etc., the civic authorities will issue CC.
Once the builder receives the Completion Certificate, they have to obtain Occupation Certificate from the civic bodies.
OC certifies that the building has complied with all the required building standards, local laws and it is safe to occupy. Occupancy certificate is issued by local municipal authorities or building proposal department that provides no objection to occupy the building under reference for its specified use. The OC is issued only once the building has been completed in all respects and can be occupied.