From my past question, I understand that each attack requires an attack roll 1d20+mod, but does this extend to reactions such as Attack of Opportunity as well?
The PHB didn't seem to say anything about this.
Page 195 of the Player's Handbook says:
To make [an] opportunity attack, you use your reaction to make one melee attack against the provoking creature.
Thus when you make an opportunity attack, you are making a regular melee attack and use all the normal rules for making one; the fact that it is being made as a reaction is not relevant.