Hi
rajangupta ,
Welcome to an awesome community

I think this is the best decision to start so early and get through GMAT hurdle.
Let me provide the answers to your query.
a) Which books to use ..Offline or on line .. Would like to have first basics ( OG) and than advanced from Man or Kap ?
Offline: Buy latest version of Official Guide. Buy Manhattan guides(soft copy or hard copy).
Online: I believe
Magoosh material is the cheapest out there to get through the basics. Once you know basics and strategies, its all about applying them to to the right number of questions.
b) When do i give the Mock exam ?
This should be your First step. Stop thinking about anything and just download the free version of GMATPrep from
www.mba.com and give your first mock religiously.
This will help you understand your current level, your weaknesses and how to customise your study plan accordingly.
c) Would like to finish up OG in 15 days and than appear for Mock ?
I would not prefer this. It's better to go with what I suggested in above question and then start going through the basics.
Keep us posted about your scored and we can help you decide the next steps.
All the best