Greetings. We'll have a 5 hour layover in the late afternoon of April 17. Is that time enough to leave the airport for some sightseeing? Thanks.
You can tour the charming town of Jessheim to kill time which is less then 20 minutes away from Oslo airport.
Have in mind that there may be additional time needed to enter Schengen zone first (visa?) and leaving it again. Depends on your starting and destination point of journey.
There is a dedicated train that goes from the airport into Oslo. It is expensive though.