All such businesses must maintain books of accounts, comply with statutory audit requirements and submit income tax returns and annual filings with the RoC.
There is no difference in capital requirement between an OPC and a private limited company. It needs an authorised capital of ₹1 lakh to begin with, but none of this actually needs to be paid-up. This means that you don’t really need to invest any money into the business.
No general advantages; though some industry-specific advantages are available. Tax is to be paid at a flat rate of 30% on profits, Dividend distribution tax applies, as does minimum alternate tax.
An OPC has certain limitations. The person starting the business is its only director and shareholder. There is also a nominee director, but this person has no power whatsoever for raising equity funds or offering employee stock options. The nominee exists only to take over in case of the death or incapacitation of the director. The nominee is chosen by the director, and can be anyone, such as your spouse, parents or siblings. The nominee will need to provide identity proof during registration.
No, an individual can form only one OPC at a time. This rule applies to the nominee in an OPC, too.

The cost of an OPC is only marginally lower than that of a private limited company. You’ll be shelling out around ₹12,000 to incorporate, then paying around ₹15,000 a year in compliance fees and an auditor to inspect your books.