Most long distance buses arrive in Oslo Bus Terminal. There is a taxi rank by the terminal. If you want to take public transportation you have to walk to Jernbanetorget (by the Central Station). Tram number 11, 12 and 13 and bus number 30 and 31 will take you to the hostel. The stop is called Hausmannsgate.
Walking distance from Oslo Bus Terminal to the hostel: 7–8 minutes