There currently are a variety of options to travel by night train from Hyvinkää (Finland) to Oulu (Finland), taking between approximately 8 and 10 hours: Namely the Yöjuna 267, the Yöjuna 269 and the Yöjuna 273. 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.