Taking into account the local weather in Oshima-gun Yamato-son can help you maximize your time at the pool during your trip. The hottest months are usually August and July with an average temp of 80°F, while the coldest months are January and February with an average of 61°F. The rainiest months in Oshima-gun Yamato-son are June, May, August, and September, with each month seeing an average of 13 inches of rainfall.