Østmarka is the broad forest east of Oslo, with quiet trails, small lakes, and the feeling that the city has dropped away faster than it should. Locals use it for hiking and lake swims in summer, and for cross-country skiing once winter settles in.
It is less famous than Nordmarka, which is part of the appeal. Come for an easy half-day in the woods, bring a map or a charged phone, and expect rocky paths, spruce forest, and water views rather than a single headline sight.