Programs: AA EXP 1.7 M, Hilton Gold, Hertz 5*, AARP Sophomore, 14-time Croix de Candlestick
If you could go up to NOK 1480 (weekday Web rate for a single), the Thon Opera is immediately adjacent to the Station. You exit the station via the doors that face the new Opera House, and the Thon is right there:
Anker Hostel will do what you ask for significantly less, and you can get a private room and private bathroom. Even with the fees for sheets and towels and breakfast, you`ll come out ahead. It`s not luxurious, but I can`t say I suffered staying in this clean, bright place. www.ankerhostel.no
Best Western Anker, in the same complex, is probably also cheaper. (Actually, it seems it`s just called Anker Hotel now.) I did not stay here. http://www.anker-hotel.no
Another useful bit of knowledge is that Norwegian hotel prices in the bigger cities drop markedly on Friday & Saturday nights as the Businessmen leave town.
I've been juggling dates for Oslo for 4 years to ensure I can afford the Opera Hotel which I regard as the best in Norway for the train traveller.
Perfect location, efficient friendly staff, a superb breakfast and every amenity you could desire.
But it's pretty dear Sunday through Thursday nights.
The Thon Astoria is an old reliable just a few blocks from the train station. On the more modest end of the Thon chain though it looks like they've refurbished since I last stayed there a few years ago.
I also see that Scandic is running a summer sale, which might help you. There's a booking deadline, possibly April 29? Check the Other Hotel Plans forum.