You choose multi city option you select the dates and upon return you select another flight to OSLO Norway (OSL) it must be within the 24 hours of arriving back to London, but you actually do not need to fly this last leg of the flight.
as they more reliable then Edreams Plus you can get nectar points and 1% cashback on topcashback.co.uk/
for those who didn't yet register on TCP register here.
Sample of dates i did on edreams.co.uk/
London to New York on the 05-Nov
New York to London on the 13-Nov
London to Oslo on the 14-Nov
This "method" is well documented on FlyerTalk amongst other flight forums and on all of them, rule #1 is DO NOT call the agents or airline so as to preserve this method for the future.
Booking the last leg to:
can reduce the cost even further.