Hi. An Occupancy certificate (OC) is an absolute must for any project anywhere. It indicates that the project is completed as per all norms and that there is no deviation between what the builder received permission for and what was actually constructed. So, if the builder had permission for only 12 floors and actually constructed 12 floors, he gets the OC. But someone who constructs 13 floors against permission for 12 floors, does not get OC. There are other things too, but this is the simplest explanation I could think of.
Not having an OC affects your ability to resell the flat - ask me, I am suffering in a non-OC flat. Want to sell my 2BHK and move to a 3BHK, but unable to sell for lack of OC!