Taking into account the local weather in Onøya is an important factor for planning your visit, especially if you plan to spend your trip poolside. The hottest months are usually August and July with an average temp of 53°F, while the coldest months are February and January with an average of 34°F. The rainiest months in Onøya are October, December, September, and November, with each month seeing an average of 7 inches of rainfall.