Want to explore 875-year-old monastery ruins, swim in the fjord, and spot red foxes all within a 5-minute ferry ride from downtown Oslo? Hovedøya (meaning "The Main Island") is the closest island to the city center, sitting in the Oslofjord as a protected landscape area famous for medieval Cistercian monastery ruins from 1147 and excellent swimming spots.
The substantial Cistercian monastery ruins (Hovedøya kloster) date to 1147 when English monks founded a powerful wealthy institution. The Reformation in 1532 ended this when the abbot was thrown in prison and the monastery burned down. You can walk freely through the remaining stone walls and arches. The atmosphere is particularly strong in late afternoon light.
Two main beaches serve swimmers. The large grassy lawn near the ferry dock attracts families. The better spot requires walking across the island (follow signs) to the south/west side where rocky outcrops and small sandy coves offer clearer water.
On warm summer weekends, "half of Oslo" heads to the islands. Expect packed beaches and long queues at the ferry dock.