OSL-ATH-HEL, $57, 3170 miles (75% credit = 2,377 miles), 2.4 cpm

Available select Sundays starting April 2016. Overnight in ATH.

Fare code: P0BAGTI

Samples:
https://www.kayak.com/flights/OSL-HEL/2016-04-17
https://www.kayak.com/flights/OSL-HE...05-18-flexible


Edit: It also works WAW-ATH-HEL and CPH-ATH-HEL for similar prices but slightly fewer miles.

It actually works for most city pairs (but not all) that are served by A3, with ATH always as the layover. Obviously, the further away both cities are from ATH, the more miles you get.