The automated rebooking cannot handle everything, especially since all non-SAS flights from OSL/CPH/ARN are booked out. Agents can see better availability, but are obviously swamped. The OAL have programmed their systems to show less inventory to the bot that SAS uses (easily done by setting availability to zero for the booking office ID with
Amadeus Dynamic availability) since they are not interested in the cheapo revenue they will get from SAS for the precious few seats.
You can still claim EC261 compensation even if you are rebooked, as long as you reach your destination late (the number of hours depending on the distance of the flight). And as far as I can tell it's 600 per cancellation, so if they keep "rescheduling" you on their flight the next day and then cancelling that it quickly adds up to enough to make a fight worthwhile.
No compensation if the cancellation was due to extraordinary circumstances, which includes strike. You can go to court for it, but no lawyer will touch your case for 600€ comp.