There currently are a variety of options to travel by night train from Kytömaa (Finland) to Oulu (Finland), taking between approximately 8 and 10 hours: Namely the Yöjuna 265, the Yöjuna 267, the Yöjuna 269 and the Yöjuna 273. These lines are operated by VR.
VR, Finland's national railway operator, runs passenger trains throughout the country, including overnight services to northern Finland.