Not a problem, you have a stop over not a transit (you can also checkin luggage on a transit). Your bags will be checked in by BA in OSL to LHR, then 5 days later you rock up to LHR and QF will check in your two bags - and apply any excess luggage if you are above the limit for example.

there is no option that your bags would have been checked in to SYD and stored at LHR for five days….