In short
The Nordmarka, a forest of several hundred square kilometres, begins where the metro line ends. You set off walking or skiing from a station, without a car.
The Oslofjord is crossed on an ordinary transport ticket: the ferries to the islands are part of the city network, not excursions.
What it really costs
Oslo is among the most expensive cities in Europe, and alcohol is taxed to the point of changing habits: a beer in a bar costs what a meal costs elsewhere.
The workaround is well known and universally practised: you buy at the Vinmonopolet, the state monopoly, and eat at home. Going out to dinner is an occasion, not a routine.
What nobody tells you
The right of public access — allemannsretten — lets you walk and pitch a tent for one night almost anywhere, private land included, as long as you keep away from houses.
The major museums — the Munch, the National Museum, the Viking Ship Museum — are paid and dear, while the forest, the fjord and the islands cost only a transport ticket.
In December the day lasts six hours. In June it barely gets dark. Nothing shapes the year here more than that swing.