Originally Posted by East_and_West

Yep, this is well-known. There is even a dedicated thread on FT:
Is A3 using OA flight numbers to deny *A benefits?

Understandably this can be quite frustrating the first time, since this is rather poorly communicated, especially when it is a connecting flight you booked on A3's website. You see an A3 flight number on tour booking, you get to enter your *A FFP number, and so on. Even more so when after being denied the benefits you believe you are entitled to, you find out that it's in fact an A3 metal flight, with A3 crew. I have experienced that too as SK*G. But see it as an LH flight operated by Air Dolomiti, an LX flight operated by Edelweiss, a TG flight operated by Thai Smiles, etc.

As a rule of thumb:
-Greek domestic flights on OA (A3) - no benefits for non-A3*Gs
-Intra-European/international domestic flights on A3 (OA) - *G benefits for non-A3*Gs