Cheapest priced copy (Lorwyn): $0.35 at Card Kingdom, near mint, their price list as of .
Sentry Oak 
Creature — Treefolk Warrior · 3/5 · Uncommon · Lorwyn · #38
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.)
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Lorwyn | 38 | Uncommon | $0.35 | $0.49 |
One ruling from Wizards of the Coast
- 1 October 2007Specifically, this ability triggers when the beginning of combat step starts during your turn.