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When Oath of Teferi enters the battlefield, exile another target permanent you control. Return it to the battlefield under its owner's control at the beginning of the next end step.
You may activate the loyalty abilities of planeswalkers you control twice each turn rather than only once.

When Oblivion Ring enters the battlefield, exile another target nonland permanent.
When Oblivion Ring leaves the battlefield, return the exiled card to the battlefield under its owner's control.

Flash
Lifelink
When Obscura Interceptor enters the battlefield, it connives. When it connives this way, return up to one target spell to its owner's hand.

When Obscura Polymorphist enters the battlefield, exile up to one target creature. Its controller seeks a creature card.

When Obscura Storefront enters the battlefield, sacrifice it. When you do, search your library for a basic Plains, Island, or Swamp card, put it onto the battlefield tapped, then shuffle and you gain 1 life.

When Obsessive Skinner enters the battlefield, put a +1/+1 counter on target creature.
Delirium — At the beginning of each opponent's upkeep, if there are four or more card types among cards in your graveyard, put a +1/+1 counter on target creature.

This spell costs {1} less to cast for each land your opponents control that could produce {C}.
Flying
When Obsidian Charmaw enters the battlefield, destroy target nonbasic land an opponent controls.

When Obstinate Baloth enters the battlefield, you gain 4 life.
If a spell or ability an opponent controls causes you to discard Obstinate Baloth, put it onto the battlefield instead of putting it into your graveyard.

When Obzedat, Ghost Council enters the battlefield, target opponent loses 2 life and you gain 2 life.
At the beginning of your end step, you may exile Obzedat. If you do, return it to the battlefield under its owner's control at the beginning of your next upkeep. It gains haste.

Flying
When Oceanus Dragon enters the battlefield, tap target creature an opponent controls. Goad it.

When Octo Opus enters the battlefield, 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.

When Octoprophet enters the battlefield, scry 2.

When Odric, Blood-Cursed enters the battlefield, create X Blood tokens, where X is the number of abilities from among flying, first strike, double strike, deathtouch, haste, hexproof, indestructible, lifelink, menace, reach, trample, and vigilance found among creatures you control.

When Odunos River Trawler enters the battlefield, return target enchantment creature card from your graveyard to your hand.
{W}, Sacrifice Odunos River Trawler: Return target enchantment creature card from your graveyard to your hand.

Disguise {4}{R}
When Offender at Large enters the battlefield or is turned face up, up to one target creature gets +2/+0 until end of turn.

When Ogre Arsonist enters the battlefield, destroy target land.

When Ogre Gatecrasher enters the battlefield, destroy target creature with defender.

When Ogre Savant enters the battlefield, if {U} was spent to cast it, return target creature to its owner's hand.

When Oil-Gorger Troll enters the battlefield, you gain 3 life. Then if you control a permanent with an oil counter on it, draw a card.

When Oji, the Exquisite Blade enters the battlefield, you gain 2 life and scry 2.
Whenever you cast your second spell each turn, exile up to one target creature you control, then return it to the battlefield under its owner's control.

When Ol' Buzzbark enters the battlefield, roll X six-sided dice onto the battlefield from a height of at least X inches. For each die, put a number of +1/+1 counters equal to the result on each creature you control that die is touching. For each die, Ol' Buzzbark deals damage equal to the result to each creature an opponent controls that die is touching.

Trample
Other creatures you control have trample.
When Old One Eye enters the battlefield, create a 5/5 green Tyranid creature token.
Fast Healing — At the beginning of your precombat main phase, you may discard two cards. If you do, return Old One Eye from your graveyard to your hand.

When Old Rutstein enters the battlefield or at the beginning of your upkeep, mill a card. If a land card is milled this way, create a Treasure token. If a creature card is milled this way, create a 1/1 green Insect creature token. If a noncreature, nonland card is milled this way, create a Blood token.

When Old-Growth Dryads enters the battlefield, each opponent may search their library for a basic land card, put it onto the battlefield tapped, then shuffle.

When Oltec Archaeologists enters the battlefield, choose one —
• Return target artifact card from your graveyard to your hand.
• Scry 3.