msem-instigator

When Ornery Bull enters, return a creature you control to its owner's hand.

Defender
Whenever another creature you control enters, Orphan of the Nav gets +1/+1 until end of turn and can attack as though it didn't have defender this turn.

Whenever you make first contact, Oscuran Wingmate gains flying until end of turn. (You make first contact the first time each turn you cast a spell with a name not in your graveyard or control already.)

Lifelink
At the beginning of your upkeep, Osgaerith Desecrator deals 1 damage to up to one target creature.

Deception {B} ({B}, Return an unblocked attacker you control to hand: Put this card onto the battlefield from your hand tapped and attacking.)
When Oshiruko Operative enters, target player loses 1 life.

Deception {2}{U} ({2}{U}, Return an unblocked attacker you control to hand: Put this card onto the battlefield from your hand tapped and attacking.)
When Oshiruko Silencer enters, return target creature to its owner's hand.

Distorted (This creature can be blocked by tapped creatures but not untapped creatures.)

When Osseous Augur enters, target creature an opponent controls gets -1/-1 until end of turn. If that creature's controller is cursed, that creature gets -3/-3 until end of turn instead.

Reveal the top four cards of your library. You may put a creature or land card from among them into your hand. Put the rest into your graveyard.

Enchant player
At the beginning of enchanted player's upkeep, they tap an untapped creature they control.

Out-of-Depth Amateur can't attack or block alone.
Attain {4}{R} ({4}{R}: Transform this. Spend at least three colors of mana to activate this, and only as a sorcery.)

{1}{W}, Exile Outcast of the Folded Fan from your graveyard: Create a 1/1 white Bird creature token with flying. Activate only as a sorcery.

Outer Wall Guard enters with a +1/+1 counter on it if you gained life this turn.

Outfitted Ibex has menace as long as it's embattled. (A creature is embattled if it has a counter on it or a permanent you control attached to it.)

Destroy target attacking or blocking creature.

Flash
When Outland Alchemist enters, return another permanent you control to its owner's hand.

Vigilance, lifelink

Outlands Bodyguard can't attack alone.
You may exert Outlands Bodyguard as it attacks. When you do, it must be blocked this turn if able. (An exerted creature won't untap during your next untap step.)

Prowess
Foresight — When Outpost Merchant enters, if you've drawn two or more cards this turn, create two 1/1 white Bird creature tokens with flying.

Prowess
Whenever Outsider Alchemist deals combat damage to a player, create a Vial token. (It's an artifact with “{1}, Sacrifice this and another permanent: Draw a card. Activate only during your turn.”)

Lifelink
Outskirts Provider enters with a +1/+1 counter on it if a creature attacked this turn and wasn't blocked.

Hexproof
When Overburdened Trader enters, create three 1/1 colorless Construct artifact creature tokens.

Counter target spell. For each other card named Overcharge in your graveyard, create a colorless Canister artifact token with “{T}, Sacrifice this artifact: Add {C}.”

When Overconfident Batter dies, if it's enchanted or you cast a spell that targeted it this turn, you gain 3 life.

Target creature gets +2/+1 and gains haste until end of turn.
Agonize (You may cast this card from your graveyard by putting a -1/-1 counter on a creature you control in addition to paying its other costs. Then exile this card.)