there is nothing that is terribly close to osaka, you would need to take a trip down the the wakayama breaks. these beaches, like most in japan, are very busy. they get all the osaka people coming down. however if you go out in the wet season with all the storms there will only be surfers out getting on the big swell. if you are up for a few hours travel in a different direction than shikoku is a good place to hit, but really a car is best for hitting either of these two places so you can get somewhere away from the crowds. check out this:
Japansurf.com - Everything you need to know about surfing in Japan!