why is the weather in the north of norway considerably warmer than oslo in the south at the moment?

Answer 1

Yes, it is much colder in south Norway, I live in Tønsberg.
The reason is, as explained by the other answered what south Norway and a good deal of northern Europe is right now under a very high pressure. I read 1045 hPa here and that's high! In the meantime, north Norway is under the influence of a low pressure moving north and bringing mild air from the Norwegian sea that is warmed up by the Gulfstream.
In the south, the lack of clouds and air transport makes the surface to cool down by radiating heat toward outer space. But at the same time, the sinking air above is warmed up by adiabatic effect. Two days ago, when I checked it, the temperature at the surface was -6 C but at 5,000 ft above it was +5 C! That is 11 degrees warmer! In the mountains, you also get a warmer temperature on the tops than in the valleys. That's because cold air is denser and sinks in the valleys. Sometimes that sinking is so violent that it creates winds up to gale force as sometimes seen in Greenland.
Unfortunately, the inversion (warm air on top of cold air) also creates fog near large bodies of water, as the Oslo fjord where I live. I had hoped to fly my small aircraft from Jarsberg last week end, alas, freezing fog is a no-no when it comes to flying. Today, the fog is gone but ... I am at work.

Answer 2

Oslo is inland (well almost) and in the east of the country, where it is influenced by the cold, settled central European anticyclone (high-pressure) area at this time of the year.

Trondheim, although a lot further north, is influenced largely by the prevailing weather from the west (Atlantic); the airflow from this direction is warmed over the warmer waters of the Gulf Stream oceanic current.

Answer 3

You should check out a map of weather flow pwtterns. Just like the ocean with undersea currents and rivers. For instance; the water temp. in Bar Harbor Maine is much colder than in Boston just a few miles away. The reason is that the icy cold gulf stream weaves its way all over the place. Going close to maine, but missing boston. The atmospheric currents are no different.

Answer 4

Theres probably a lot more cloud over northern Norway, whereas Oslo probably has clear skies, allowing the heat to escape.
It's been the same here in the UK for a while. In London its 3 degrees, but in Stornoway in Scotland its 10 degrees.

Answer 5

There's a Scandinavian High at the moment and it's making S norway and UK as well as other places very cold.

This is a blocking high and tends to persist