We have a gnome fighter and a dragonborn bard in our campaign. The gnome sometimes gets in a makeshift sling on the dragonborn's back to shoot arrows at enemies during combat. Thus far I've considered the dragonborn an independent mount when opportunity attacks (OAs) arise, and have also granted the gnome cover as applicable.
Recently the dragonborn received a horse. Now the gnome is on the dragonborn's back and the dragonborn is on the horse. If the horse is a controlled mount and it provokes an OA then the attacker can choose to attack the horse or the dragonborn. On mounts the PHB states (198):
… if the mount provokes an opportunity attack while you’re on it, the attacker can target you or the mount.
This seemed straightforward at first but then we realized the dragonborn, not the horse, is the gnome's mount. However the dragonborn will never provoke an opportunity attack on its own because the rule on opportunity attacks states (PHB 195):
You also don’t provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction.
In this case neither the dragonborn nor the gnome can provoke an opportunity attack because they are not moving using their movement, action, or reaction. Only the horse can provoke an opportunity attack.
If the attacker cannot attack the gnome in this case, then the gnome is completely safe from OAs.
If the horse provokes an opportunity attack can the attacker decide between the dragonborn, the horse, and the gnome?