There definitely is an Athens -Instanbul segment supported by Oneworld carriers, its just not a direct flight. I flew this segement ( in reverse) on a RTW award ticket last year. Using Malev Air - Istanbul - Budapest - Athens or alterntively another option is with Iberia - Istanbul -Madrid-Athens. Unfortunately the connection times are crap - if i remember correctly, I arrived from Istanbul at 6:30am and had to wait approx 5 hours for my connecting flight to Athens. ( This was May last year, so the timetable has probably changed)

I agree with Kiwi Flyer, you need to search the segements separately.