Well, buying an apartment which does not have an Occupancy Certificate has many challenges associated with it. An Occupancy Certificate (OC) is one of the most important things to look for before buying an apartment. An apartment complex is eligible for an OC only if it has been built as per the approved building plan and has followed all the building bye-laws.
One of the biggest implication of not having an OC is whether the property is safe to inhabit. The builder could have done any amount of deviation making it unsafe to move-in. Only if you do a Civil Engineering inspection of the apartment complex, will you be able to identify the deviation.
The second biggest concern is that the property will not get "A Khata" without an OC. Suppose, tomorrow if you plan to sell this apartment the same banks which gave you loan today will not give it to your prospective buyer because the property does not have "A Khata".