Of course it's best for SAS having all the 330s and 340s at one place. But now they fly out of both ARN and CPH. I don't see any reason why SAS shouldn't fly out of OSL. In fact, it makes more sense to move both ARN-EWR and ARN-ORD to OSL since it is.. ehhh on the way???
Besides C checks and other heavy maintenance, there's no problems operating OSL-ORD with only one AC, fly out of OSL at 1015, land at ORD 1200, return 1530 and land back at OSL at 0645.
I mean, C checks take a couple of weeks, for the AC to get to the maintenance base at ARN or CPH and to get the replacement AC from these bases to OSL takes 50 minutes..
SAS has all the equipment needed to serve the 330 at OSL already