There are 3 ex OSL flights on Monday according to the App

07.25
16.45
20.15

I would anticipate that the desks in the morning would close around 06.40 as the staff woud then move to the gate to do gate ops there.

BA desks won't then reopen until check-in opens for the 16.45 so around 14.45 based on opening 2 hours before departute but could be a little earilier.