I assume you bought your ticket in OSL?
Check the situation in Norway where you have the contract with the airline. In several European countries are court rulings that make it difficult/impossible for airlines to charge extra if you fly not all the flights on your ticket. If that is the case you are safe.
Does anybody knows what happens when you get sick on your flight HKG-LHR - something like food poisoning and you need to go to a hospital? Would the airline offload your luggage or return it to LHR?