There currently are a variety of options to travel by night train from Helsinki (Finland) to Oulu (Finland), taking between approximately 8 and 11 hours: Namely the Yöjuna 273, the Yöjuna 265, the Yöjuna 267 and the Yöjuna 269. These lines are operated by VR.
VR, Finland's national railway operator, runs passenger trains throughout the country, including overnight services to northern Finland.