NOC REQUIREMENT

We are planning to purchase one apptt. There it is compulsory requirement to obtain NOC from registered A. O. P. for registration of property. How can registrar enforce and prevent any purchaser for want of NOC? How A. O. P. can compel a govt. authority, to prevent registration of property on full payment of Stamp Duty and Registration fees by buyer? Is there any control of A. O. P on Registrar? If yes, under which clause of which law?