I am a newer DM just getting I to DnD with the latest edition here. One of the players in my group has taken the role of fighter and using the human varient he started with the polearm mastery feat which allows him to make an opportunity attack on creatures who enter his range, and add an additional 1D4 damage bonus action attack after his attack roll.
Originally he was using this incorectly by making the bonus attack with the opportunity attack as well but we found that he could only do that with his actual attack action. The new question we have is if he's supposed to add his proficiency and str modifier to his bonus attack damage as he would his normal attack.