I had a related question. I am using a quarterstaff, one handed, with a shield. Nothing in the feat description leads me to believe I cannot do this and I should still gain all the benefits of the feat. Somehow though it does not feel right? Can someone realistically wield a quarterstaff one handed with a shield in their off-hand and hit you effectively with both ends? Isn't the idea of a polearm to keep your enemies at bay and strike the first with your longer reach before they can engage. It feel the quarterstaff could definitely benefit from the bonus action attack if wielded two handed, regardless of range, but the opportunity attack seems like a stretch if they aren't considered reach weapons.
This came up because I liked playing pacifist clerics in V4 but there really is no such thing in V5 except perhaps the life cleric. I liked the idea of not making offensive attacks, but being able to defend myself if someone came at me with ill intent. This feat as a life cleric was the perfect fit for that concept.
Thoughts?
I have two separate thoughts to address aspects of this.
1) I like to think of weapons as categories, more than defined weapons. A quarterstaff as a versatile weapon that does 1d6/1d8 bludgeoning damage seems totally reasonable. However, a large baseball bat or heavy bokken would clearly do more damage than a basic club even one-handed and could be wielded two handed to even greater effect; as such it would also be appropriate as a versatile weapon that does 1d6/1d8 bludgeoning damage. Since the cost, weight, damage, and tags are all the same, there is no need for a different weapon line item in the PHB, "quarterstaff" is used for either. The mental image of a quarterstaff and shield just seems jarring to me, but baseball bat and shield seems like it would be quite effective, especially in a zombie apocalypse. ;)
2) I agree "the idea of a polearm to keep your enemies at bay and strike the first with your longer reach before they can engage." To reference my former point, that is definitely not true of a baseball bat. However, it could easily be said that this feat, as with feats in general, is not so much a reflection of the wielded weapon as the training and mindset of the character. I would say meaningful parallels could be drawn between this feat and the techniques of Iaijutsu (the rapid drawing strike for the Katana), colonial musketmen ("don't fire until you see the whites of their eyes"), or a pacifist life cleric ("I don't start fights, I end them - quickly!"). Someone who specializes in polearms is not someone who seeks to trade blow-for-blow and outmatch their opponent in a "fair" battle. Someone who specializes in polearms is someone who gambles on injuring their opponent in the very first instant of engagement; preferably when asymmetric weapon reach and a defensive posture allows the polearm master to land a blow that could define the fight before their opponent is even effectively engaged. It is all about waiting for the precise instant when they can land a blow and seizing that moment before their opponent has the chance to land theirs. Of course, sometimes an opponent will get past that sweet spot and get inside a polearm wielder's preferred range, where the asymmetric ranges favor the opponent. In that circumstance, being able to fend off their opponent with the haft or butt of the weapon could well save the polearm master's life. Obviously, the haft is never going to do as much damage as the part intended intended for striking, but that is why this feat puts it to 1d4 - basic club damage. Thinking of this feat as a trait of the character and not of the weapon means a polearm specialist is always watching for the opportunity to get in that perfect first strike or that quick haft/butt/hilt jab, regardless of the weapon they are using.
The need to pick and choose which weapons appear in the PHB weapons list, and which are compatible with the feat is obviously both necessary and somewhat arbitrary; it cannot be both simple enough for the game to flow and nuanced enough for "realism". A 5e pseudosamurai can't get a preemptive opportunity attack with a longsword flavored as a katana but can with a quarterstaff flavored as a bokken if they have this feat; that is arbitrary and imperfect, but that is the nature of rules. That is why the designers are clear they - themselves - value Rulingsover Rules. If you have a character concept that would require bending the rules a little but the result would be fun without being unbalanced - talk to your GM, that is why they exist. If your DM is not comfortable with it, try to accept it and move on to a different method or concept; there are plenty of ways to skin a tabaxi in 5E, but pretty much the only way to "lose" D&D is if you make the DM your enemy.
1st question: Does that mean 10ft? My reach is 10ft, so that should be the case, correct? My 2nd question is about the regular opportunity attack. When a creature leaves my reach. What then? Is my reach still 10ft? On my character page it says that the reach for my opportunity attack (Polearm master - opportunity attack) is just 5ft.
I have no idea. And on a sidenote, it also says that the bonus attack is just 5ft. So, anyone that knows more than me, please help!
While you are wielding a glaive, halberd, pike, quarterstaff, or spear, other creatures provoke an opportunity attack from you when they enter your reach, or move out of it. The Opportunity Attack and bonus action attack has whatever reach the weapon has, and let you add strenght modifier to the damage roll.
You can jab and swing with a quarterstaff. When I say a jab, it's like a thrust with spear and it's devastating. I took a martial arts class and our teacher would spar with students in Kendo Armor using a staff. He could jab with the tip of the staff and hit you in the face five times before you even knew what happened.
I can't help but necro this thread, because I 100% agree with you.
I think people are discounting how much pain a good jab with a small, blunt object can cause. Imagine a punch, but with the same force spread across a much smaller area than a fist. It 100% makes sense that a quarterstaff's stab would do damage with a jab.
Additionally, there are people in this thread saying it makes no sense that someone could wield a shield and still make 2 attacks with a quarterstaff. Those people have no imagination.
You can 100% wield a staff in it's center (Like how a bo staff is typically held) and make two strikes with both ends exceptionally fast by spinning the staff around. It wouldn't even get in the way of the shield as you could spin it overhead or off to one side and you would still be protected by said shield as seen in the extremely realistic drawing I did with my amazing photo-manipulation skills below. The same goes for a short spear.
TL;DR: PAM should 100% be allowed to be utilized with a shield and spear/staff, not JUST because it's RAW, but also because you can literally actually attack with both ends of a spear/staff while holding a shield. Try it with a broom handle. I just did and it doesn't take much imagination to realize it'd work.
I have two separate thoughts to address aspects of this.
1) I like to think of weapons as categories, more than defined weapons. A quarterstaff as a versatile weapon that does 1d6/1d8 bludgeoning damage seems totally reasonable. However, a large baseball bat or heavy bokken would clearly do more damage than a basic club even one-handed and could be wielded two handed to even greater effect; as such it would also be appropriate as a versatile weapon that does 1d6/1d8 bludgeoning damage. Since the cost, weight, damage, and tags are all the same, there is no need for a different weapon line item in the PHB, "quarterstaff" is used for either. The mental image of a quarterstaff and shield just seems jarring to me, but baseball bat and shield seems like it would be quite effective, especially in a zombie apocalypse. ;)
2) I agree "the idea of a polearm to keep your enemies at bay and strike the first with your longer reach before they can engage." To reference my former point, that is definitely not true of a baseball bat. However, it could easily be said that this feat, as with feats in general, is not so much a reflection of the wielded weapon as the training and mindset of the character. I would say meaningful parallels could be drawn between this feat and the techniques of Iaijutsu (the rapid drawing strike for the Katana), colonial musketmen ("don't fire until you see the whites of their eyes"), or a pacifist life cleric ("I don't start fights, I end them - quickly!"). Someone who specializes in polearms is not someone who seeks to trade blow-for-blow and outmatch their opponent in a "fair" battle. Someone who specializes in polearms is someone who gambles on injuring their opponent in the very first instant of engagement; preferably when asymmetric weapon reach and a defensive posture allows the polearm master to land a blow that could define the fight before their opponent is even effectively engaged. It is all about waiting for the precise instant when they can land a blow and seizing that moment before their opponent has the chance to land theirs. Of course, sometimes an opponent will get past that sweet spot and get inside a polearm wielder's preferred range, where the asymmetric ranges favor the opponent. In that circumstance, being able to fend off their opponent with the haft or butt of the weapon could well save the polearm master's life. Obviously, the haft is never going to do as much damage as the part intended intended for striking, but that is why this feat puts it to 1d4 - basic club damage. Thinking of this feat as a trait of the character and not of the weapon means a polearm specialist is always watching for the opportunity to get in that perfect first strike or that quick haft/butt/hilt jab, regardless of the weapon they are using.
The need to pick and choose which weapons appear in the PHB weapons list, and which are compatible with the feat is obviously both necessary and somewhat arbitrary; it cannot be both simple enough for the game to flow and nuanced enough for "realism". A 5e pseudosamurai can't get a preemptive opportunity attack with a longsword flavored as a katana but can with a quarterstaff flavored as a bokken if they have this feat; that is arbitrary and imperfect, but that is the nature of rules. That is why the designers are clear they - themselves - value Rulings over Rules. If you have a character concept that would require bending the rules a little but the result would be fun without being unbalanced - talk to your GM, that is why they exist. If your DM is not comfortable with it, try to accept it and move on to a different method or concept; there are plenty of ways to skin a tabaxi in 5E, but pretty much the only way to "lose" D&D is if you make the DM your enemy.
While you are wielding a glaive, halberd, pike, quarterstaff, or spear, other creatures provoke an opportunity attack from you when they enter your reach, or move out of it. The Opportunity Attack and bonus action attack has whatever reach the weapon has, and let you add strenght modifier to the damage roll.
Spear One-handed, Reach 5: 1d6 + STR piercing / 1d4 + STR bludgeoning (bonus action)
Spear Two-handed, Reach 5: 1d8 + STR piercing / 1d4 + STR bludgeoning (bonus action)
Quarterstaff One-handed, Reach 5: 1d6 + STR bludgeoning / 1d4 + STR bludgeoning (bonus action)
Quarterstaff Two-handed, Reach 5: 1d8 + STR bludgeoning / 1d4 + STR bludgeoning (bonus action)
Glaive Two-handed. Reach 10: 1d10 + STR slashing / 1d4 + STR bludgeoning (bonus action)
Halberd Two-handed. Reach 10: 1d10 + STR slashing / 1d4 + STR bludgeoning (bonus action)
Pike Two-handed. Reach 10: 1d10 + STR piercing / 1d4 + STR bludgeoning (bonus action)
You can jab and swing with a quarterstaff. When I say a jab, it's like a thrust with spear and it's devastating. I took a martial arts class and our teacher would spar with students in Kendo Armor using a staff. He could jab with the tip of the staff and hit you in the face five times before you even knew what happened.
I can't help but necro this thread, because I 100% agree with you.
I think people are discounting how much pain a good jab with a small, blunt object can cause. Imagine a punch, but with the same force spread across a much smaller area than a fist. It 100% makes sense that a quarterstaff's stab would do damage with a jab.
Additionally, there are people in this thread saying it makes no sense that someone could wield a shield and still make 2 attacks with a quarterstaff. Those people have no imagination.

You can 100% wield a staff in it's center (Like how a bo staff is typically held) and make two strikes with both ends exceptionally fast by spinning the staff around. It wouldn't even get in the way of the shield as you could spin it overhead or off to one side and you would still be protected by said shield as seen in the extremely realistic drawing I did with my amazing photo-manipulation skills below. The same goes for a short spear.
TL;DR: PAM should 100% be allowed to be utilized with a shield and spear/staff, not JUST because it's RAW, but also because you can literally actually attack with both ends of a spear/staff while holding a shield. Try it with a broom handle. I just did and it doesn't take much imagination to realize it'd work.