It depends. If the LHR-OSL tickets were bought separately from the OSL-BOO, then they may be out of luck. SAS wouldn't have necessarily known that your parents were inbound on BA and not originating in OSL. That's always the risk of buying two separate tickets for one connecting journey.
If however, they bought them on one itinerary from BA, then they can't be held accountable for the delay out of LHR, and they should have been rolled over to the next flight. If that's the case, then keep pursuing it with BA.