As [MENTION=9540]Dave Noble[/MENTION] said above, most important is to check QR ticket. If it has some flex, I would be less worried. If it does not have flex, and given that it is November where weather can happen both at LHR and OSL, I would definitely leave more than 3 hours unless you have a flex ticket. There have been some previous QR stories this winter and last on FT. Some ended bad (new ticket) and some ended better. But QR is under no obligation to change your ticket out of goodwill, so I would maybe even consider an overnight and explore Oslo, but 3hrs is a tad tight