Pretty much by definition all heirloom plants are open-pollinated, but not all open-pollinated plants are heirlooms.

Exactly what "heirloom" means depends on who you ask, but by and large heirlooms are older varieties of open-pollinated plants that have been conserved by repeatedly growing them out again and again over the years.

New varieties of open pollinated plants are being developed even now. If they manage to stick around for twenty, thirty, forty, fifty years or more they too can become heirlooms.

.....Alan.