Weather is probably a major factor when you plan your trip to Onøya, and keep in mind that cheaper options are usually available when the weather isn't as good. 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.