I was unable to get the UA missing-mileage credit page to accept an Aegean number, operated by Olympic, flight. Booked through Aegean website, and the ATH-HER confirmation said A3 7320 operated by Olympic Air. Ticket stock was 390 (Aegean). However my boarding pass listed the flight # as OA 320. So I'm guessing this may not work, as United generally looks at the operating carrier for determining credit.