In november mid week return flights London Stansted to oslo can be found for £4. Ryanair now fly to gardemoen which is oslos main airport and is 25 mins by train and costs £10 each way to the city. I recommend buying a oslo pass for £30 a day which gives access to all museums and public transport. The viglands statue park is always free and is the best tourist attraction in oslo. Dont forgot it will be cold in Oslo. Alcohol and restaurants are expensive so dont forgot to bring buy some tax free.