Subject to no big delay, and having checked recent incoming flight from Oslo stats: here you are most likely be fine. Not quite sure where you got the idea of closing boarding early. At Helsinki it usually only starts 25-30 min till departure time for a short-haul flight (A320). Although travelling to London will make you pass from EU (Norway is non eu but in Shengen, like Iceland or Switzerland) to non-shengen (UK) zone, so investigate while at OSL may be the departure gate for your next flight be known and study the HEL map. they may even announce them in the OSL-HEL plane. And run, if necessary

p.s. you are returning on Sunday before noon, and Finns are still asleep at that time, and definitely not travelling much. Joking aside, the lines at passport control and security will be short.