There are several options to get to Arendal from Oslo. The bus Vy190 takes 3.5 hours to arrive, there is a change at Harebakken which takes you to Tinghuset right next to Arendal. Another bus Nw192 from Filipstad in Oslo takes a little longer but takes you directly to the bus station in Arendal. Otherwise, train 50 takes 5h20. The fastest option is by car, which takes about 3h.