Oath of Druids — ruling 1

The ability can only target an opponent of the current player.

Oath of Druids — ruling 2

The targeted player controlling more creatures than the current player is a part of the targeting requirement. A player can't be targeted by this ability unless it's true, and the ability doesn't resolve if it's no longer true at that time.

Oath of Druids : règles, fonctionnement et interactions (MTG)

Oath of Druids is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

At the beginning of each player's upkeep, that player chooses target player who controls more creatures than they do and is their opponent. The first player may reveal cards from the top of their library until they reveal a creature card. If the first player does, that player puts that card onto the battlefield and all other cards revealed this way into their graveyard.

How it works : Oath of Druids

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2009· Wizards of the Coast
The ability can only target an opponent of the current player.
No other card is named in this ruling.
#2October 1, 2009· Wizards of the Coast
The targeted player controlling more creatures than the current player is a part of the targeting requirement. A player can't be targeted by this ability unless it's true, and the ability doesn't resolve if it's no longer true at that time.
No other card is named in this ruling.