My pre-trip research has revealed that several significant Oslo sights are not currently open. Those planning trips should take this into consideration in deciding how much time they need in the city.
The Viking Ship Museum will be closed until 2026. I believe some items from the museum are on display at the National Historical Museum.
The National Gallery, which Rick labels the country's best art museum, has been closed for some time. The website says it will reopen this year, but no specifics are given, so I wouldn't count on it for a summer 2022 trip.
The Parliament Building (Stortinget) is offering no tours (and thus no public access) at the moment. I believe this is related to COVID-19.
All operating hours should be checked; many places are closing an hour earlier than older guidebooks indicated.
On a happier note: Those with trips coming up soon should check out the Wednesday 6 PM - 9 PM free-entry period offered through June at the Edvard Munch Museum. The usual entry charge is NOK 160, about US $18, and there's no senior discount. I know we're all gun-shy about taking advantage of free-access periods in museums that could be unpleasantly crowded. I haven't been to that museum, but the website makes it appear a relatively modern, spacious place, so I think it could be OK even during a free period. Note that not all of Norway's Munch works are in this museum. Some are part of the (currently closed) National Gallery's collection; others are in the KODE in Bergen.
There's a special Munch exhibition scheduled for the Courtauld Institute in London from May 27 until September 5; I suspect some of the Norwegian National Gallery's Munch paintings are being loaned for that exhibition. That's one reason I don't expect the Norwegian Museum to reopen in the next few months--but I'm only guessing here.