You two are talking about slightly different things. According to wikipedia, Oslo has:
- * 6.4C as the maximum temperature at any time in January of a given year then averaged over 1991-2020.
- * 0.1C as the maximum average daily temperature for any day in January of a given year then averaged over 1991-2020.
- * -4.7C as the minimum average daily temperature for any day in January of a given year then averaged over 1991-2020.
- * -13.8C as the minimum temperature at any time in January of a given year then averaged over 1991-2020.
For comparison, Minneapolis (north of Chicago) has:
- * 5.8C as the maximum temperature at any time in January of a given year then averaged over 1991-2020.
- * -4.7C as the maximum average daily temperature for any day in January of a given year then averaged over 1991-2020.
- * -12.9C as the minimum average daily temperature for any day in January of a given year then averaged over 1991-2020.
- * -25.9C as the minimum temperature at any time in January of a given year then averaged over 1991-2020.
So the north of the USA does seem to have a colder climate than the south of Norway (Oslo). That definitely does not hold true for other parts of the USA like southern California so your mileage may vary.