Sentry Oak

Creature - Treefolk Warrior 3/5, 4W (5)
Defender

At the beginning of combat on your turn, you may clash with an opponent. If you win, Sentry Oak gets +2/+0 and loses defender until end of turn. (Each clashing player reveals the top card of his or her library, then puts that card on the top or bottom. A player wins if his or her card had a higher converted mana cost.)
Illus. Warren Mahy

Gatherer Card Rulings [Nov 2009]

• 10/1/2007: Specifically, this ability triggers when the beginning of combat step starts during your turn.

Format Legality	Legal in Vintage, Legacy, Extended, MTGO Classic Banned in Standard
Printings (1/10)	Lorwyn (Uncommon)