If you’re hoping to get lost in the rain on your romantic getaway, check out these details about year-round temperatures in Oswego: The hottest months are usually August and July with an average temp of 67°F, while the coldest months are February and January with an average of 30°F. The snowiest months in Oswego are January, February, March, and December, with each month seeing an average of 14 inches of snowfall.