In a city like Oslo with prices as high as its snow drifts, the Thon Astoria offers good value for money. It´s 5 minutes from the central railway station and bus terminal and handily placed for most everything the tourist or business traveller on a budget would want. Part of a chain, they can recommend other local sister hotels.
As always, I recommend getting travel agant ID (www.sctravelteam.com) It´s easy and inexpensive and it will pay for itself many times over.