Lanefan
Victoria Rules
That's very poor wording in the PH. A simple amendment to read "...creature that you can see attempts to move out of your reach..." ("begins to" or "tries to" work just as well) would make it all make much more sense both in the fiction and as a series of sentences.Opportunity attack PH Page 195
"You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To
make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach."
It is not when they are "moving" out of your reach, it is when the creature moves out of your reach.
If they are within your reach, that is within your reach and before they move out of your reach.
That is my natural language reading of this 5e rules section
If you hit them with a grapple they are restrained and do not actually move out of your reach.
Which is all fine except when the blocking tries to happen after the swing has already been declared to have hit, because that declaration makes it too late to undo. Ret-con effects like this really are awful* design.Except in the fiction reaction spells are just as quick as fighting style interrupt actions. Blocking someone's blow with a quick defensive spell seems as fictionally genre appropriate as blocking someone's swing with a fighting style bodyguard type interrupt action.
* - believe me, I'd use much stronger words here if Eric's Grandma wasn't listening...
