There currently are a variety of options to travel by night train from Oulu (Finland) to Iittala (Finland), taking between approximately 8 and 9 hours: Namely the Yöjuna 264, the Yöjuna 266 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.