Around 50 per cent of buildings in Mumbai don’t have OC 

2) Since the building does not have an OC, it is not recognised by the fire brigade.you have to pay higher property taxes 

3) The Brihanmumbai Municipal Corporation (BMC) can prosecute the residents for unauthorised occupation, but only courts can decide their fate, which means the residents can stay until a verdict comes in

4) don’t purchase the flat 

5) it is not considered regularised due to age 

6) banks generally are reluctant to sanction loan for purchase of flats in building without OC 

You can buy

Many do that and live in such buildings without any problems

However some banks may not sanction home loan

The building also cannot be termed as deemed regular 

There are many buildings in Mumbai without an OC which have the essential services like water and electricity in place

Though under law it is illegal to occupy a building without OC but in many cases the Municipal Corporation takes no action and things move on like that for decades

1) There is no problem in buying the flat without OC.

2) Yes, okay to buy as society is formed and you can suggest to society body to get OC regularised it meanwhile.

3) You need to check with flat owner whether deemed conveyance is done or not otherwise that too can be get by applying to state government as there is scheme going on by State govt.

4) For such a old building Bank do not sanctioned loan or mortgage, once building is passed 40 years above.

1. Generally without OC no amenity service would be provided,  I.e., electricity and water supply etc.

Since the flat is having all the facilities you may obtain a legal opinion from a local lawyer and proceed if recommended. 

2. In my opinion there should not be any problem especially if a registered society is functioning under which this flat falls. 

3. You may obtain a legal opinion about this aspect too.

4. The bank may consider various factors to sanction the loan to satisfy its requirements/conditions.