I checked the fastest train connection from Oulu to Helsinki and it was 5 hours 27 minutes. The slowest was 9 hours 32 minutes but that was a night train.

The fastest bus connection was 8 hours 10 minutes. (Horrible, if you ask me.)

Of course, you have to add travel time from home to Oulu railway station to the total and pretty much only that if your destination is Central Helsinki.

I've flown from Tampere to Helsinki as a child. Our destination was Vancouver, BC. There were two stops: Helsinki and London. The flight between Tampere and Helsinki was quite short, only about 25 minutes or something like that.