The ruined foundations of St. Olav's Abbey lie in the lower town near the waterfront, largely overlooked by visitors who don't know to look for them. The abbey was founded by the Franciscans in the 13th century and became one of the important religious houses on the Oslofjord coast. It was dissolved during the Reformation in the 16th century and subsequently dismantled, its stone reused in other buildings.
What remains is fragmentary but historically significant: the abbey church once stood here, and the site preserves some of the few visible medieval remains in the lower town outside Slottsfjellet. Access is free. The ruins sit in a small public space and are easy to combine with a walk along the waterfront.