Average Temperatures in Oslo, Norway. 1  The average annual temperature in Oslo, Norway is cool at 5.7 degrees Celsius (42.3 degrees Fahrenheit). 2  There is a variation of average monthly temperatures of 20.7 C (37.3 F) which is a below moderate range.3  There is a range/ variation of mean daily temperatures of 7.3 C (13.2 F). There is a range/ variation of mean daily temperatures of 7.3 C (13.2 F). 2  The warmest month (July) is very mild with an average temperature of 16.4 degrees Celsius (61.52 degrees Fahrenheit). 3  The coldest month (January) is slightly cold having a mean temperature of -4.3 degrees Celsius (24.26 degrees Fahrenheit).