There is indeed a lot to do in Oslo!
“Oslo” translated from old Norwegian language means: ‘the valley of Gods”.
If you going to have a half-day sightseeing, I would recommend you to have:
For example: a city tour by foot for 3 hours, showing the city center, i.e. Aker Brygge, the Royal Palace, the Parliament, the University, the main streets, etc. You will also visit a medieval Fortress ‘Akershus’.