What to do if builder is not getting OC certificate ?

Hi Sir, This is here. We have selected a ready to move flat to purchase, and already paid the booking amount. The builder has mentioned that they have already applied for OC, and they will write in agreement to provide the same. However, when I checked the acknowledgement, it's been already one year passed. I believe, they should have received the OC by this time. Could you please let me know you point of view? And also, if there is any way to know the reason behind this delay.