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?
Best Answer
Yes
The Two-handed property definition on pg. 147 (after errata 1.0) has the easy answer:
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:
and the wording of unarmed strike:
there isn't anything preventing you from making an unarmed strike while holding your weapon, even if both hands were full.