oak fern (plural oak ferns)
- Any of various ferns which grow on trees and walls
- genus Gymnocarpium (syn Thelypteris)
- especially common polypody (Polypodium virginianum, native to North America, and Polypodium vulgare, native to Europe), formerly thought to have medicinal properties when found growing on an oak.
- 1603, John Florio, translating Michel de Montaigne, Essays, II.12:
- Why say we, that […] to know the vertue of Reubarb, qualitie of Oak ferne, and operation of Polipodie, is only peculiar unto man?