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 (Druid, Ninja, Alchemist, in the respective order).