An OC or Occupancy Certificate is given to a property when it has been constructed as per the approved building plan from the government and when the builder has adhered to all the building bye-laws applicable. It essentially ensures that the building is fit to be inhabited.
You should remember that OC is not given to individual flats but to entire projects. Whereas, A-Khata is given to individual flats only and only if the project has got an OC.