There currently are a variety of options to travel by night train from Oulu (Finland) to Oulunkylä (Finland), taking between approximately 9 and 11 hours: Namely the Yöjuna 266, the Yöjuna 274, the Yöjuna 264 and the Yöjuna 276. These lines are operated by VR.
VR is Finland's national railway company, operating passenger trains across Finland, including night trains connecting the south of the country with the north.