The Trysil-Knut rittet, also known as Osensjøen rundt, is a 75 km recreational mountain bike marathon held at Sjøenden, at the southern tip of Osensjøen in Søre Osen, Trysil Municipality. Søre Osen IL organizes it annually, with the start and finish located in the same lakeside area. The main race is timed, and while mountain bikes are recommended, the route is suitable for riders comfortable on mixed surfaces. Beginners can complete it, though the climbs are demanding.
The course circles Osensjøen, covering approximately 70 km of gravel and asphalt roads, along with about 5 km of flow trail. It utilizes gravel roads, paved sections, farm tracks, and basic trails. Participants will ride near the lake's windmill installation, and a specific climb features a prize at its summit. An alternative gravel and hard-surface course is available for those preferring less varied terrain. Results from the Birken and Cykel-Vasan races can be used for seeding, attracting both club riders and experienced marathon cyclists alongside recreational participants aiming for a long ride around the lake.