It took a little time to find, but there are some postal drop boxes available at Oslo. These should be relatively easy to find and both are pre-security.
Look for a mail drop pre-security on the departures level prior to the security checkpoint; it’s left of the SAS/Star Alliance service desk, near a set of bathrooms and behind the store “Point”.
There is another mail drop on the arrivals level past the designated “meeting point” and next to an Airport Information Center, close to an exit.
Last updated 16 July 2017