So if we assume normal O5e rules, only the attacker would get an OA here.
Attacker Presses the Attack
Defender falls back. Assuming this was at the edge of the attacker's reach, this would move you outside of that reach, triggering an OA.
Attacker than moves in to complete the attack. As this is a movement within reach, it should not provoke an OA (this is a specific O5e change to OAs, different than 4e and 3e, which worked when you moved through reach). Even if it did, the defender has already used their reaction to fall back, so they wouldn't get an OA regardless.