Shouldn't be that much. If you don't mind sharing, what route did you specify and where did you obtain the price? It sounds suspiciously like you've gotten the ex-UK fare, not the ex-Norway one.

Note that relying on the Oneworld online tool for this can result in hair loss at a minimum. The tool is hopelessly buggy and unreliable, and tends to default to higher-cost options, e.g. choosing British Airways as the initial carrier in order to get BA's usurious surcharges and fees applied to all flights, not just those operated by BA. There are workarounds possible, with the emphasis on "work."