Ekeberg Restaurant is in a park on the eastern fringe of the city.
It's notable for two things.
First, the physical plant. It's a notable example of Functionalist architecture from the 1930s. Like many building projects in that decade, it was never quite finished (in this case, only some details were left undone). The restaurant was popular for a long time, then fell into desuetude, then was fairly recently renovated (the incomplete parts being completed). Now, it's popular again.
Second, the site. The restaurant is in a park, on a high bluff overlooking the Oslo fjord, offering sweeping views.
This seems like a touristy spot, but most of my fellow patrons appeared to be locals.
You wouldn't expect good food here; it would be like expecting good food in Tavern on the Green. So it's a pleasant surprise that the food is well better than adequate. I had, for lunch, the local standard shrimp toast, a pile of small shrimp on a buttery piece of toast with mayonnaise squiggled on top. These weren't the best shrimp I've had on this trip to Oslo -- but they were still at the very top of what we get in New York.
The blueberry cobbler I had for dessert suffered from being too fancied up. Sometimes trying to serve "good cuisine" gets in the way.
Despite one's natural misgivings, this place is well worth visiting. (Ekeberg Park, with its modern sculptures strewn about and its miles of hiking trails, is a worthwhile destination in itself.)
Oddly, this is operated by the same restaurant group that runs Maaemo.