Where can I buy train ticket online to travel from Osaka to Tokyo? Any URL or Website? Thanks.?

You can check timetables, prices and buy tickets;

http://www.japanrail.com/

Good luck

To purchase an actual ticket (not a rail pass) and make a train reservation you have to do it inside Japan at any JR (Japan Railways) Station reservation office.

Purchasing online and reserving online outside of Japan is not possible at the moment. Again this applies to actual train tickets and not any sort of rail pass.