Oathbreaker is played with vintage legal cards. That means the 25 cards with the “Conspiracy” card type, silver bordered cards, skill testing cards such as falling star, and the 9 cards that reference “playing for ante” are not legal. Additionally, there is a Ban List maintained by the Oathbreaker Rules Committee that can be found at weirdcards.org/oathbreaker-ban-list.
Only one copy of any given non-basic card is allowed in your deck construction. Any number of basics are allowed within the deck size. Some cards (e.g. Relentless Rats) may have rules text that overrides this restriction.
Deck size is exactly 60 cards. This includes your Oathbreaker and your Oathbreaker’s signature spell.
Players life totals will begin the game at 20.
A Planeswalker card in your deck is considered your Oathbreaker. This card is a Planeswalker card that will exist in the command zone and can be cast subject to normal timing restrictions. You may cast it again from the command zone with an additional cost of 2 generic mana for each time it has been cast from the command zone. This is an additional cost. Note you may have multiple Oathbreakers if they are using the partner game mechanic.
Whenever your Oathbreaker changes zones you may return it to the command zone instead.
Cards in a deck may not have any colors in their color identity which are not shared with the Oathbreaker of the deck. A card’s color identity is its color plus the color of any mana symbols in the card’s rules text. A card’s color identity is established before the game begins, and cannot be changed by game effects. (The identity of each card in the deck must be a subset of the Oathbreaker’s)
In addition, each Oathbreaker will have a "Signature Spell" that will also exist in the command zone. That Oathbreaker's "Signature Spell" must be an instant or sorcery card that falls within their color identity. (Note: Colorless spells are allowed for any Oathbreaker.) While your Oathbreaker is on the battlefield under your control, you may cast it from the command zone with an additional cost of 2 generic mana for each time it has been cast from the command zone this game. This is an additional cost, and does apply even if an alternative cost is paid (e.g. Force of Will).
If your Oathbreaker’s signature spell would go to any zone other than the command zone or the stack, then it instead goes to the command zone. This takes precedence over other replacement effects such as buyback.
Your Oathbreaker’s signature spell may only be cast if your Oathbreaker is on the battlefield under your control.
Oathbreakers are subject to the legend rule; a player cannot control more than one legend with the same name.
Abilities which refer to other cards owned outside the game (i.e.: Wishes, Spawnsire, Research, Ring of Ma’ruf) do not function in Oathbreaker without prior agreement on their scope from the playgroup.
If an effect refers to your “Commander” then it instead refers to your “Oathbreaker”.
Cards become legal for Oathbreaker on the date of a set's prerelease. In the event there is no prerelease, cards become legal on the date of a new set's release.
These rules were written to be simple to read, and not require a significant rules knowledge to interpret them. If you would like to see these rules as as they would look in the Magic The Gathering: Comprehensive Rules, you’re in luck! Some awesome Magic Judges and members of the Oathbreaker community have translated these rules into that format. If you’re interested, you can check it out over at OathbreakerMTG.org/rules.