Addressing a nuance in the PH errata: the rule lets melee weapon attacks use unarmed strikes, despite those strikes not being weapons.
— Jeremy Crawford (@JeremyECrawford) June 10, 2015
@Arksorn1Is intent that monk stunning strike which req “hit…with a melee weapon attack” will not work with unarmed strike?
You can make a melee weapon attack with an unarmed strike, so a monk can use Stunning Strike with an unarmed strike.
— Jeremy Crawford (@JeremyECrawford) June 11, 2015
Is a monk’s stunning strike able to be used as an opportunity attack
Yes — there is no timing requirement (during your turn, etc.) for using Stunning Strike, so you can use it whenever you make a melee weapon attack (which an OA is).