We flew dublin-oslo rygge ryanair last july. Got really cheap flights, €184 for 2 adults, 2 kids, 1 infant. Bus right outside airport and lovely drive into city itself. weather was roasting. We were there 16 days before the bombings.
We stayed in Thon Hotel Austoria, right in city centre, great value €90 a night, breakfast included, of which we made a packed lunch from (seen everyone else doing the first day) as food is expensive in Oslo(They do do free coffee refills and cups are huge, we used to order coffee/tea and drinks 4 kids and no-one ever minded us opening up our picnics outside the cafe tables (in fact all people seemed to do this, approx €14 for drinks), evening meal roughly €60/70 including one dessert. Some Tripadvisor say hotel suffers from road noise etc but I can honestly say we never heard a thing and we had family front room.
We bought the Oslo Pass (it comes with a great booklet telling u how to get there etc) for three days before we went and its fantastic value. Two outdoor swimming pools, plenty of museums, walking tours, reptile park, all city travel on buses, trams, underground. Start the Pass at 1 oclock first day and so you spread it over four days.
People are so friendly and helpful,....ENJOY.