A reach weapon adds 5 feet to your reach making it 10 feet.
If you are holding a 2-handed reach weapon (eg a glaive) with 2 hands and someone moves from 5 feet away to 10 feet away do you still get an opportunity attack? Or because you are holding the weapon are you unable to use an unarmed strike for opportunity attacks?
The Two-handed property definition on pg. 147 (after errata 1.0) has the easy answer:
This property is only relevant when you attack with the weapon, not when you simply hold it.
If you aren't using your two-handed weapon for the OA, you can just make an unarmed punch with one of your hands.
Additionally,
Given the wording of Opportunity Attacks on pg. 195:
...you use your reaction to one melee attack against the provoking creature.
and the wording of unarmed strike:
Instead of using a weapon to make a melee weapon attack, you can use an unarmed strike: a punch, kick, head-butt, or similar forceful blow (None of which count as weapons).
there isn't anything preventing you from making an unarmed strike while holding your weapon, even if both hands were full.
I would rule that if you end your turn wielding the glaive, then you can only make attacks of opportunity with the glaive. So if an enemy moves out of your reach (10 ft. with the glaive) then you can make an OA.
