Ryanair has launched a flash sale for the next three hours only (i.e. until 6pm GMT, 16 February).
In short, you can fly from London (Stansted) to Oslo (Rygge and Torp) for just £9.99 each way, including all taxes and fees.
At the time of writing, availability is good:
(Photo: “Akrobaten gang og sykkelbro” by Knut Arne Gjertsen – Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons)