Unfortunately one of the pit falls of buying a separate ticket
Unfortunately this scenario wouldn't even be covered by EU261. This was a schedule change months in advance.
The AMS-OSL - while non-refundable, was it also non-changeable? I would have expected SQ to reroute you to AMS FOC, but perhaps changing to a later connection to OSL on your original ticket.
Can your travel insurance help out?
I'd certainly pursue this with SQ, without getting your hopes up.
I can't see this being an issue for commercial ticket pax. Economy and business class are not affected by the change, leaving only a couple of F class pax that might have booked at this stage. And out of them I don't know how many would have booked a non-changeable onward connection to other Euro ports.