If you want to save money on accommodation in Oslo, consider staying in hostels instead of hotels.
The average cost of a double room in a hostel is 54 EUR per night (270 EUR for 5 days), and you should expect to pay around 28-42 EUR for a single bed in a 2- to 4-bed room (consider Oslo Youth Hostel Haraldsheim as an option). Please note that prices for accommodation in Oslo may not include breakfast and bed linen, for which you may have to pay separately. You can check out Hostelling International Norway and VIP Backpackers to find the right hostel for you.