FYI this fare is still going ex Scandinavia to Australia. Open jaw ex ARN back to OSL is a good option at SEK21,891 = 2,500 USD. To or from CPH is about $300 more than ARN/OSL.
Whichever, you need to take the ARN-DME-SIN flight in one direction at least to get this fare.
SYD flights are mostly gone but there's plenty for BNE and MEL, and PER is wide open. Check the 7-day price display on the SQ site which also works for the open-jaws.
Oh and the tickets I thought I had bought on Opodo.se? Never happened. Book direct on the SQ site!