RAW 5e, the goblins can’t be targeted by an OA since they are on the mount right? https:// PHB pg. 198 "In either case, if the mount provokes an opportunity attack while you're on it, the attacker can target you or the mount." 😁
— Dan Dillon (@Dan_Dillon_1) May 14, 2020
As a DM I’d still say they would be immune to the OA unless the attacker had the ability to actually reach them.