Sentry Oak is a uncommon Creature — Treefolk Warrior card from Lorwyn (LRW), collector number 38. Current market price: $0.18 (foil $0.35), TCGplayer market prices via Scryfall.
Defender At the beginning of combat on your turn, you may clash with an opponent. If you win, this creature gets +2/+0 and loses defender until end of turn. (Each clashing player reveals the top card of their library, then puts that card on their choice of the top or bottom. A player wins if their card had a greater mana value.)