Whenever an opponent draws a card, Ob Nixilis, the Hate-Twisted deals 1 damage to that player.

−2: Destroy target creature. Its controller draws two cards.

, : You gain 5 life.

, : Draw a card, then discard a card.

, : Target creature gets -2/-2 until end of turn.

, : Obelisk of Alara deals 3 damage to target player or planeswalker.

, : Target creature gets +4/+4 until end of turn.

: Add , , or .

: Add , , or .

, : Return target permanent you both own and control to your hand.

, : Put a fate counter on target permanent.

, , Sacrifice Oblivion Stone: Destroy each nonland permanent without a fate counter on it, then remove all fate counters from all permanents.

Flying

, Return a land you control to its owner's hand: Target creature gets -X/-0 until end of turn, where X is the number of cards in your hand.

Flying

: Obscura Initiate gains lifelink until end of turn.

: Draw a card, then discard a card.

, , Sacrifice Obsessive Stitcher: Return target creature card from your graveyard to the battlefield.

: Put a blaze counter on target land without a blaze counter on it. For as long as that land has a blaze counter on it, it has "At the beginning of your upkeep, this land deals 1 damage to you." (The land continues to burn after Obsidian Fireheart has left the battlefield.)

When Octo Opus enters, create a 4/4 blue Octopus Performer creature token named Contortionist with "At the beginning of your upkeep, you may fold this token in half." (It becomes 2/2, then 1/1, ½/½, and so on.)

Sacrifice Octo Opus: Draw a card and an additional card for each of Contortionist's folds.

At the beginning of each combat, creatures you control gain banding until end of turn if a creature you control has banding. The same is true for changeling, devoid, fear, flanking, horsemanship, ingest, intimidate, landwalk, shroud, tantrum, wither, and the activated ability "Sacrifice this creature: Add ."

When Odunos River Trawler enters, return target enchantment creature card from your graveyard to your hand.

, Sacrifice Odunos River Trawler: Return target enchantment creature card from your graveyard to your hand.

Infect (This creature deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)

: Ogre Menial gets +1/+0 until end of turn.

: Destroy target creature that was dealt damage this turn.

: Target snow creature gains first strike until end of turn. ( can be paid with one mana from a snow source.)

Double strike, indestructible

Oketra the True can't attack or block unless you control at least three other creatures.

: Create a 1/1 white Warrior creature token with vigilance.

Ninjutsu (, Return an unblocked attacker you control to hand: Put this card onto the battlefield from your hand tapped and attacking.)

Whenever Okiba-Gang Shinobi deals combat damage to a player, that player discards two cards.

At the beginning of combat on your turn, Oko, the Ringleader becomes a copy of up to one target creature you control until end of turn, except he has hexproof.

+1: Draw two cards. If you've committed a crime this turn, discard a card. Otherwise, discard two cards.

−1: Create a 3/3 green Elk creature token.

−5: For each other nonland permanent you control, create a token that's a copy of that permanent.

+2: Create a Food token. (It's an artifact with ", , Sacrifice this artifact: You gain 3 life.")

+1: Target artifact or creature loses all abilities and becomes a green Elk creature with base power and toughness 3/3.

−5: Exchange control of target artifact or creature you control and target creature an opponent controls with power 3 or less.

, : Tap target creature without reminder text. (Reminder text is still any italicized text in parentheses that explains rules you already know.)

Old-Growth Grove enters tapped.

: Add or .

Trample

Whenever Oliphaunt attacks, another target creature you control gets +2/+0 and gains trample until end of turn.

Mountaincycling (, Discard this card: Search your library for a Mountain card, reveal it, put it into your hand, then shuffle.)

Flying, lifelink

Whenever one or more outlaws you control deal combat damage to a player, create a Treasure token. (Assassins, Mercenaries, Pirates, Rogues, and Warlocks are outlaws.)

, Sacrifice two Treasures: Put two +1/+1 counters on each creature you control. Activate only as a sorcery.

Flying

Olivia's Bloodsworn can't block.

: Target Vampire gains haste until end of turn.

Vigilance

Sacrifice Ollenbock Escort: Target creature you control with a +1/+1 counter on it gains lifelink and indestructible until end of turn.

Flash

When Omen of the Dead enters, return target creature card from your graveyard to your hand.

, Sacrifice Omen of the Dead: Scry 2.

Flash

When Omen of the Hunt enters, you may search your library for a basic land card, put it onto the battlefield tapped, then shuffle.

, Sacrifice Omen of the Hunt: Scry 2.

Flash

When Omen of the Sun enters, create two 1/1 white Human Soldier creature tokens and you gain 2 life.

, Sacrifice Omen of the Sun: Scry 2.

: Add .

, , Exile Ominous Cemetery: Target creature's owner shuffles it into their library.

Whenever you draw a card, put a foreshadow counter on Ominous Seas.

Remove eight foreshadow counters from Ominous Seas: Create an 8/8 blue Kraken creature token.

Cycling (, Discard this card: Draw a card.)

, : You may cast an instant or sorcery spell from your hand without paying its mana cost.

O-Naginata can be attached only to a creature with power 3 or greater.

Equipped creature gets +3/+0 and has trample.

Equip (: Attach to target creature you control. Equip only as a sorcery.)

Creatures can't attack planeswalkers you control unless their controller pays for each creature they control that's attacking a planeswalker you control.

, Exile Onakke Oathkeeper from your graveyard: Return target planeswalker card from your graveyard to the battlefield.

Cohort — , Tap an untapped Ally you control: You gain 2 life.

Whenever one or more creatures you control deal combat damage to a player, investigate. (Create a Clue token. It's an artifact with ", Sacrifice this artifact: Draw a card.")

, Exile a creature card from your graveyard: Investigate. You gain 2 life.

: Target player loses 1 life.

Flying

: Choose a color. Target opponent exiles the top X cards of their library. For each card of the chosen color exiled this way, create a 1/1 blue and black Faerie Rogue creature token with flying.

, Remove one or more +1/+1 counters from among creatures you control: Create an X/X green Ooze creature token, where X is the number of +1/+1 counters removed this way.

Whenever an opponent casts a creature spell, if Opal Acrolith is an enchantment, Opal Acrolith becomes a 2/4 Soldier creature.

: Opal Acrolith becomes an enchantment.

Defender (This creature can't attack.)

Bushido 1 (Whenever this creature blocks or becomes blocked, it gets +1/+1 until end of turn.)

: The next time a source of your choice would deal damage this turn, that damage is dealt to Opal-Eye, Konda's Yojimbo instead.

: Prevent the next 1 damage that would be dealt to Opal-Eye this turn.

: Add one mana of any color.

Tap an untapped creature you control: Tap target artifact, creature, or land.

Opulent Palace enters tapped.

: Add , , or .

Devoid (This card has no color.)

, Put a card an opponent owns from exile into that player's graveyard: Draw a card, then discard a card.

: All damage that would be dealt to target creature this turn by a source of your choice is dealt to Oracle's Attendants instead.

, : Exile the top card of your library. Until end of turn, you may play that card. Put a brick counter on Oracle's Vault.

: Exile the top card of your library. Until end of turn, you may play that card without paying its mana cost. Activate only if there are three or more brick counters on Oracle's Vault.

Oran-Rief, the Vastwood enters tapped.

: Add .

: Put a +1/+1 counter on each green creature that entered this turn.

Ascend (If you control ten or more permanents, you get the city's blessing for the rest of the game.)

: Add .

, Sacrifice Orazca Relic: You gain 3 life and draw a card. Activate only if you have the city's blessing.

: Add one mana of any color.

Other noncreature artifacts are mono and continuous. (Each activated ability of mono artifacts costs an additional to activate if its cost doesn't already include . As long as a continuous artifact is tapped, it loses all abilities.)