Flash (You may cast this spell any time you could cast an instant.)
Reach
When Great Oak Guardian enters the battlefield, creatures target player controls get +2/+2 until end of turn. Untap them.
Any creatures controlled by the player that are already untapped will just get +2/+2 until end of turn.
Great Oak Guardian's last ability targets only the player, not any creatures. Creatures with shroud that player controls will be affected, for example.