Oslo to Helsinki is an internal Schengen flight. You would need to be in the international transit area at HEL to be out of Schengen.

So - to stay within the validity period of your visa you would need to check in for your onward flight immediately on arrival at HEL, and immediately proceed through passport control into the international area of the airport (i.e., get stamped out of Schengen before midnight)

I don't know if you can check in 21 hours before (though on a through itinerary, with through-checked baggage, this wouldn't be an issue), and I don't know if you can stay in the international departure area overnight. You should try to find out about these things well ahead of time so that you can make alternative arrangements if necessary.

Maybe someone who knows HEL can provide more information about the practicalities.