There currently are a variety of options to travel by night train from Oulu (Finland) to Korso (Finland), taking between approximately 9 and 11 hours: Namely the Yöjuna 276, the Yöjuna 274, the Yöjuna 266 and the Yöjuna 264. These lines are operated by VR.
VR, Finland's national railway operator, runs passenger trains throughout the country, including overnight services to northern Finland.