Can Colossus Slayer add damage on Attack of Opportunity?

@mikemearls @JeremyECrawfordColossus Slayer says you can add damage to an attack made once per turn. Can it be used on AoOs? Or to other additional attacks made on another creatures turn, when you used it during your attack?

— Jason Wasson (@jwasson5226) August 27, 2016

@jwasson5226 @JeremyECrawford usable only if the attack takes place during your turn, so no AoOs, etc unless they happen during your turn

— (((Mike Mearls))) (@mikemearls) August 29, 2016