Hi
1) You can proceed to buy the property without Occupancy certificate as there are other documents such as Sale deed, Khata certificate, Khata Extract, EC, Latest tax paid receipt, conversion order etc.
2) Also since you there is an approved plan, Occupancy certificate should not be a problem.
3) Also suggest you to take an actual measurement of the apartment and check whether the flat dimensions(built up area, carpet area of the flat) are as per approved plan.
4) If there are deviations in the dimensions between actual at site and approved plan, then better to renegotiate the price with the current seller as you will need to pay the regularisation charges from your pocket.
5) if there are no deviations, please take an indemnity bond from the seller that in the event of any penalties being levied by BBMP that the seller will indemnify you in case of any claims on account of regularisation/penalisation by BBMP.
Looks like you have all the documents to go ahead for the purchase. Points 3, 4 and 5 are outlined by us only for providing a sense of comfort/as a matter of abundant caution.
Hope this helps