Planning for the weather is a good way to make your trip to Oyama relaxing and enjoyable, especially when you're splurging. The hottest months are usually August and July, with an average temperature of 21°C, while the coldest months are January and February, with an average of 4°C. The rainiest months in Oyama are July, September, October and June, with each month seeing an average of 274 mm of rainfall.