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