Cheapest priced copy (Commander 2019): $0.35 at Card Kingdom, near mint, their price list as of .
Great Oak Guardian 
Creature — Treefolk · 4/5 · Uncommon · Commander 2015 · #37
Flash (You may cast this spell any time you could cast an instant.)
Reach
When this creature enters, creatures target player controls get +2/+2 until end of turn. Untap them.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
6 printings · $0.35 to $0.69 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2015 | 37 | Uncommon | $0.69 | — |
| Commander 2019 | 170 | Uncommon | $0.35 | — |
| Commander Anthology | 113 | Uncommon | $0.59 | — |
| Tales of Middle-earth Commander | 247 | Uncommon | $0.49 | — |
| The List | C19-170 | Uncommon | $0.35 | — |
| Starter Commander Decks | 186 | Uncommon | $0.35 | — |
2 rulings from Wizards of the Coast
- 4 November 2015Any creatures controlled by the player that are already untapped will just get +2/+2 until end of turn.
- 4 November 2015Great Oak Guardian's last ability targets only the player, not any creatures. Creatures with shroud that player controls will be affected, for example.