Traveling during Obon

I'll will be arriving in Japan on August 15 and i was wondering if it is going to be busy because i'll be using the Tokaido Shinkansen from Tokyo to Osaka. I'm just worry that there is not going to be any seats left. also ill be taking the Shinkansen around 10am - noon

I think it will probably be busy, so try reserving a seat as soon as you arrive, and if you can't find any seat at the time you want, you can always just get a non-reserved seat and line up to board the train.
Worst-case scenario, you will have to let a few trains pass, or you can always stand all the way (really).

What if i went on green class will it also be busy?

Probably yes...

I checked the vacancy on Cyberstation

http://www.jr.cyberstation.ne.jp/vacancy/Vacancy.html

and found that on 15th Aug, 10:00 to 12:00 all of Shinkansen including Hikari have vacant seats. The peak should come on 11th when there is no vacancy at present.