Heart of Oak is the second upgrade in the middle path for the Druid in BTD6. Heart of Oak allows the Druid's thorns to strip Regrowth Bloons of their regrow property. It costs $295 on Easy, $350 on Medium, $380 on Hard and $420 on Impoppable.
Heart of Oak, similar to Counter-Espionage and Perishing Potions, removes one specific bloon property type from bloons. Coincidentally, all of these upgrades come from Magic Monkeys as the 2nd upgrade of their 2nd path (Druid, Ninja, Alchemist, in the respective order).