If OSL-LHR is the only bit cancelled then you can, I believe, ask the cancelled service to be removed, however some people have had different experiences. BAH may cause particular issues here, see further on. There is a 300 mile gateway rule on commerical bookings but it can be flexed a few extra miles. THis is going to need a call and a lot of patience, just rank your options, with a re-timed OSL as the bottom one. BA can use Lufthansa Group airlines for an indirect routing. Yes you have to call Bremen, and that may cause a problem since removing OSL then turns it into a UK BAH and I don't think they can transfer over. I wouldn't work on the basis of regulations, the contact centre staff won't enjoy that, but have your hierarchy of options lined up.