My Oslo favorite - though pretty expensive - are Vitis Vinifera at Sjolyst - "business downtown" where the consulting firms etc are.

Madserud Gard is in the same area (obviously this is where my OSL residence is..) and I like it a lot. Expensive, and the service varies...

If I go out with guests new to Norway, I would take them to Babette’s Guesthouse - where they serve Norwegian-French gourmet food. Very traditional though and again - EXPENSIVE...