Without Occupancy Certificate, your possession is not at Legal. Without OC you have taken the possesion of an unauthorised structure.
Now a days, after RERA has been introduced, things have improved on the buyer's side in these kind of matters. It would be great if you can form a Group with other members and approach RERA Tribunal. You can register a complaint online in Just Rs.5000/-.
Best would be that you all form a group and approach a good Property Lawyer.
Thanks & Regards
Advocate Abhishek Sinha