Does Polearm Master's bonus attack action proc. on an opportunity attack of a monster entering my reach?
You can't use the bonus action attack after an opportunity attack.
The extra butt-strike attack that you can make with the Polearm Mastery feat has several requirements that you can't meet in this situation. Lets look at the relevant part of the Feat's rules (from the PHB, page 168):
When you take the Attack action and attack with only a glaive, halberd, quarterstaff, or spear, you can use a bonus action to make a melee attack with the opposite end of the weapon.
First of all, the extra attack requires you to use a bonus action, and you can only make a bonus action on your own turn. You can't make a bonus action on another character's turn.
Secondly, the specific conditions that the feat imposes on the bonus action won't be met after you've made an opportunity attack. Specifically, the feat says "When you take the Attack action...", and an opportunity attack is not the same thing as the Attack action. See this previous question about the difference between the Attack action and the more general term, attack.
Polearm Master reads as follows (emphasis mine):
"When you take the Attack action and attack ..."
Since opportunity attacks are not "taking the attack action", Polearm Master does not apply. You make only the regular attack as your opportunity attack.