The Following is an analysis based on a strict reading of the rules, and ignoring designer intent and their use of language. The lead designer for 5e had this to say about Reach weapons:
Rules for Opportunity attacks and Reach weapons don't make (narrative?) sense, but they are consistent. Reach is always 5 feet when not actively making an attack, and not the weapon being used.
- As you will see from the rules of the polearm master, the purpose of opportunity attacks is mainly to prevent people from running past you, and not to punish people who flee your engagement with them.
To understand how/why this is the case, we must first work with the assumption that it's possible and logical that a person with a 10 foot reach weapon when they attack, is going to have a reach of 10 feet for the purpose of opportunity attacks.
Let's first be clear about what an opportunity attack in 5e is. Normally, an opportunity attack is an attack you are able to make when a creature leaves the area you can reach. It's a penalty for a creature leaving combat without first disengaging. It is not an attack which demonstrates what area you threaten. That is, if somebody enters your reach, or moves about within your reach, you do not threaten them. Opportunity attacks seem to serve two purposes:
- It creates a cost by way of the "disengage" action to leave melee combat with someone once it has been initiated. (This is based the application of the rules with 5 foot reach weapons, but not directly stated in the rules)
- It prevents enemies running right past you to engage with someone else you are defending/blocking. (Page 195 of the players handbook)
You can rarely move heedlessly past your foes without putting your self in danger; doin g so provokes an opportunity attack.
However, when using reach weapons, this seemingly falls apart, no matter which way you interpret the rules.
The blue square with an x is you. The purple square with an x is an ally, and the green square with an x is a medium creature with speed 30. The red square is your 5foot reach, the Orange square is your 10 foot reach.
Scenario 1: Reach for opportunity attacks is 5 feet.
In this scenario, you have just finished attacking the green monster. It is now the monster's turn. You engaged them in melee combat, but he can now step away from you and go off and do something else without any penalty. Purpose 1 of opportunity attacks has been subverted. It's arguable that this isn't really the purpose of opportunity attacks, but I think it's the basic assumption for most people, and it's consistent with the rules for 5 foot reach weapons.
Scenario 2: Reach for opportunity attacks is 10 feet.
In this scenario, you have just finished attacking the green monster. The green monster now wants to go and attack the purple ally since he is an easier target than you to hit. The monster can smoothly walk right past you, without ever leaving your 10 foot reach. This means you can never do an opportunity attack against them. Purpose 2 of the opportunity attacks has been subverted, as directly stated by the rules.
That is the situation with your run of the mill opportunity attack. But now let's include the polearm master feat. (Page 168 of the Players Handbook)
You can keep your enemies at bay with reach weapons. You gain the following benefits:
• When you take the Attack action and attack with only a glaive, halberd, or quarterstaff, you can use a bonus action to make a melee attack with the opposite end of the weapon.
The weapon's damage die for this attack is a d4, and the attack deals bludgeoning damage.
• While you are wielding a glaive, halberd, pike, or quarterstaff, other creatures provoke an opportunity attack from you when they enter your reach.
And now looks more closely at the wording of the opportunity attack rule.
Page 195 of the Players handbook
The attack interrupts the provoking creature’s
movement, occurring right before the creature leaves your reach.
If we edit the rule with polearm master in mind it would read as follows:
The attack interrupts the provoking creature’s
movement, occurring right before the creature leaves or enters your reach.
So now, with this wording, if your reach with the polearm is 10 feet, you can never actually reach the target which you are stopping. The creature will be 15 feet away from you when you interrupt it's movement before it enters your reach. However, if your reach is 5 feet, then before it enters your reach the creature is 10 feet away from you. When you attack the creature, your reach now extends to 10 feet, and you and you can hit it. This would seem to strongly suggest that two important things.
- Reach for opportunity attacks is always 5 feet, and not your weapon or attack.
- The purpose of opportunity attacks is ONLY to prevent people from running past you, and not to prevent people from running away from you after being attacked.
The alternative is assuming that the rules are wrong and lots of errata is needed all over the place to "fix it".
Something else to notice, is that none of the monsters in the DM Basic rules are listed as having a reach of 10, or anything other number. However, many attacks are listed as having a reach of 10. Such as the Adult dragons' bite. (Though the claw attack is listed as having a reach of 5). In many cases, you can see one attack having a reach of X, while another attack has a reach of y. The section in the rules which discusses the reach of a creature extending beyond 5 feet is specifically in the "Melee Attacks" section, and not in the other sections which mention reach.
Most creatures have a 5-foot reach and can thus attack targets within 5 feet of them wh en making a melee attack. Certain creatures (typically those larger than Medium ) have melee attacks with a greater reach than 5 feet, as noted in their descriptions.
Of course, if your table decides to play things out differently, you may do so, but be consistent if you do.
All that being said, in the end, the intention of the rules is that an Opportunity Attack is an attack and so the reach property applies to the conditions that allow you to attack as well. As can be seen from this conversation on Twitter.
For creatures, you will see that different attacks have different reaches, and so an Ancient Red Dragon for example will have one reach domain for it's claws and another for it's bite, giving it a greater "threat area".