Highlights of Oslo

https://www.ricksteves.com/europe/norway/oslo

Here are my to see of Rick's list:

▲▲▲ City Hall Oslo's artsy 20th-century government building, lined with huge, vibrant, municipal-themed murals, best visited with included tour.
▲▲▲ National Gallery Norway's cultural and natural essence, captured on canvas.
▲▲▲ Frogner Park Sprawling park with works by Norway's greatest sculptor, Gustav Vigeland, and the studio where he created them (now a museum).
▲▲▲ Norwegian Folk Museum Norway condensed into 150 historic buildings in a large open-air park.
▲▲ Norwegian Resistance Museum Gripping look at Norway's tumultuous WWII experience.
▲▲ Viking Ship Museum An impressive trio of ninth-century Viking ships, with exhibits on the people who built them.
▲▲ Fram Museum Captivating exhibit on the Arctic exploration ship.
▲▲ Kon-Tiki Museum Adventures of primitive Kon-Tiki and Ra II ships built by Thor Heyerdahl.
▲▲ Holmenkollen Ski Jump and Ski Museum Dizzying vista and a schuss through skiing history. (I would have this a 3 star item. Tram T1 ride to Holmenkollen is scenic).
▲ Akershus Fortress Complex and Tours Historic military base and fortified old center, with guided tours, a ho-hum castle interior, and a couple of museums (including the excellent Norwegian Resistance Museum, listed above).

If you had an extra day or two the Nutshell would be the highlight of a Norway visit.

24 hour Oislo Card may be worthwhile.

Bus 30 goes from the Oslo S area to the Bygdoy peninsula with the Folk Museum, Viking Ship, Kon Toki and Fram museums. The Fram is one of my highlight featuring the Nobel Peace Prize Arctic explorer Fridtjof Nansen. Take the boat back to the City Hall.

Thanks so much for the information!

For restaurants alone, I think it's worth picking up the latest copy of Rick's Scandinavia book. I like using a digital copy from iBooks or Kindle, that way there's no extra weight in your bag. I agree with Rick that restaurants in Oslo and Norway in general are nothing special and prices are quite high. "Egon" is a chain restaurant that my cousin took me to as their go-to "I don't want to cook tonight," and it was surprisingly good.

If you have a day with good visibility, it might be worth a trip up to Ekebergparken. There's a restaurant up there and a neat sculpture park with great views of the city.

Restaurants. A Norwegian told me that if he wanted to eat good Norwegian food he would eat at home. With this in mind, one of the better Chinese meals we had was at the Oslo "Dinner Restaurant"
http://dinner.no/nb/
Stortingsgata 22, 0161 Oslo, Norway
(Our wait person spoke American. He went to college in Oakland, California.)

I've eaten a lot of Peppes pizzas in Oslo. Not that they are gormet pizzas but the price is as reasonable as can be eating out if a first world economy. The beer can cost more than the pizza. (Stock up from the ship's dury free shop if they have one).

https://www.peppes.no/
One Peppes restaurant is across the street from the Oslo S train station.

A lot of dining options at the Aker Brygge complex including a Peppes Pizza.
http://www.akerbrygge.no/english/
http://www.visitoslo.com/en/activities-and-attractions/boroughs/aker-brygge-tjuvholmen/

Aquavit is the national drink. The Linie Aquavit has crossed the equator.
http://lordsofthedrinks.com/2015/03/31/linie-aquavit-the-norwegian-drink-that-gets-its-unique-taste-from-crossing-the-equator/

If you had more days, the Norwegian Trekking Association
Den norske turistforening, DNT mountain huts serve traditional meals. (After a week in the mountains enjoying traditional meals, Peppes Pizza tastes pretty good).

Thanks for all of the responses!