I'm pretty sure you WILL get full benefits for the first flight. It's an actual A3 flight but they use the aircraft of OA. I know it's confusing, but a good rule is that if the flight number is a "regular sequence" A3 number then it will be treated as a full A3 flight, despite the metal flown.
This is similar to somebody who posted recently about being suprised to receive full benefits in IST while flying IST-ATH on one of these OA Q400 flights.