Oslo’s seasons are defined less by temperature than by light. At 60 degrees north, the difference between June and December is not a matter of degree.
The light
Midsummer: the sun sets around 10.45pm and it never gets properly dark. Oslo is outdoors until midnight and the whole city changes character.
Midwinter: sunrise around 9.15am, sunset around 3.10pm. Under six hours of daylight, much of it grey.
Neither is a reason to avoid a season. But if you have not experienced either, plan for it — and if darkness affects your mood, do not book December.
By season
Spring (April–May). Uncertain and improving. The city comes outdoors the moment it hits about fifteen degrees, which happens abruptly. 17 May, Constitution Day, is the single best day to be in Oslo — the children’s parade up Karl Johans gate, everyone in national costume, the royal family on the balcony for hours.
Summer (June–August). The best months. June is arguably the peak: long light, the islands open, the fjord swimmable, and everything working. July is the Norwegian holiday — the fellesferie — so the city empties, some restaurants close, and hotel prices fall. Temperatures are usually high teens to mid-twenties, occasionally much more, and most buildings have no air conditioning.
Autumn (September–October). September is genuinely good — clear, the forest turning, and the cultural season starting. October is greying. November is the worst month: dark, wet, no snow yet.
Winter (December–March). Cold, typically well below freezing, with reliable snow from January.
This is when the forest justifies the city. Around 2,600 km of prepared cross-country trails, about 90 km floodlit, reached on the metro. Sledging at Korketrekkeren. The Holmenkollen Ski Festival in March.
December has the Christmas markets at Spikersuppa and Norsk Folkemuseum, and the Nobel Peace Prize on the 10th, when the centre is given over to the ceremony and the torchlight procession.
Prices
Oslo is a business city, so midweek is the expensive time and weekends are cheaper — the reverse of most leisure destinations.
July is cheap because Norwegian business stops.
Cheapest overall: January, February and November.
What to wear
Whatever the season, a waterproof and a layer. Oslo is wet and the weather turns fast.
Winter: proper insulated boots with grip. Pavements ice over and the city grits rather than clears. Norwegians use brodder, strap-on studs, and they are sold in every supermarket for very little.
Summer: shorts and a fleece, in the same bag.
The one date to know
17 May. If you can arrange to be in Oslo for it, do. If you are trying to book a hotel for that weekend without planning, you will struggle.
The longer version is friluftsliv, which explains where everyone goes at the weekend in every season.