HI gmatwithpooja,
The last few OGs have a number of things in common with one another:
The OG13 and GMAT2015 are the exact same book, but the GMAT2015 includes some online resources that allow you access to the questions in the book in a different way.
The GMAT2016 includes 25% 'new' content (meaning that 25% of the questions from the GMAT2015 have been 'swapped out'; this does NOT mean that the GMAT has significantly changed the material that will show up on Test Day - the 'swapped out questions' are still relevant).
In answer to your general question, ANY of these books will be useful during your studies, but you do not necessarily need to purchase more than one of them. The older versions of the OG are likely a bit less relevant, but they are still reasonable practice resources (and they're not that expensive).
GMAT assassins aren't born, they're made,
Rich