As Parnel notes, the website is hopeless for anything but basic AC award flight bookings and the occasional simple return AC/STAR itinerary. You will have to call AE and pay the $30 fee (I don't think non-Canadians get charged the GST) to make the booking you'reafter. However, I am not sure on an intra-regional STAR award/or even one between Europe's or Asia's two zones, that a stopover and open jaw are permitted...even a stopover. At least I was told this by an AE agent when I wanted to do a SIN-ICN s/o ICN-NRT destination/turnaround NRT-SIN routing. (But remember, AE agents are not infallible and more often than not give incorrect information themselves.)

Note also that there is just a 5% overage allowed when it comes to Maximum Permitted Mileage. So the return segment cannot exceed the distance of the original flights by more than 5% on an award ticket, not the 25% that is permitted on a revenue one. Though I have not calculated the mileage of your itinerary, you might also want to do this before calling.

Whether these are correct assumptions, these are problems I have encountered when making more complex STAR award redemptions.

Otherwise, I think you should have no problem making the booking with an agent. (Hey, I couldn't even find OSL as a destination in the pulldown airport menu on the AE site...)