msem-instigator

Flying
When Old-World Forgotten dies, you may return target artifact or enchantment card from your graveyard to your hand.

Haste (This creature can attack and {T} as soon as it comes under your control.)
Olson's Sky Brigade has flying as long as you control another attacking Pirate.

Flash
When Olten Vipermoa enters, you may return another creature you control to its owner's hand. If you do, put two +1/+1 counters on Olten Vipermoa.

Colossal 3—{5}{U} (You may cast this for its colossal cost to have it enter with three +1/+1 counters; or pay {5}{U} as a sorcery: Put three +1/+1 counters on it if it has none.)
When Omendiver's colossal ability puts one or more counters on it, scry 3.

Flying
Whenever Omid enters or one or more Armies you control attack, recruit. (Create a 0/0 white Soldier Army creature token, then put a +1/+1 counter on each Army you control.)

Ominous Dragoncaller gets +1/+1 and has flying as long as you control a Dragon.

Vigilance (Attacking doesn't cause this creature to tap.)
Sacrifice a Fungus, Saproling, Plant, or Insect: Put a +1/+1 counter on Ontarachie Forager.

Lifelink
Whenever a planeswalker dies, Onya's Chosen deals 3 damage to target creature or player.

Flying
Spells your opponents cast cost an additional 2 life to cast.

When Oozing Yaoguai enters or a creature you control scavenges it, up to one target creature gets -3/-3 until end of turn and you gain 3 life.
Scavenge {3}{B}{G} ({3}{B}{G}, Exile this card from your graveyard: Put a number of +1/+1 counters equal to this card's power on target creature. Scavenge only as a sorcery.)

Flying
At the beginning of your end step, up to one other target creature you control voyages. (If it isn't blocking or targeted by another spell or ability, it phases out.)
Whenever a creature you control phases in, you may have it become a copy of target creature you control until end of turn.

Ophan Titan gets +3/+3 as long as there are no nonland cards in your graveyard.
Morph {1}{G} (You may cast this card face down as a 2/2 creature for {3}. Turn it face up any time for its morph cost.)

Voracious 2 (As this enters, you may sacrifice up to two artifacts and/or creatures. This creature enters with that many +1/+1 counters on it.)
When Opportunistic Beast enters, destroy up to one target creature that was dealt damage this turn.

Bleed {1}{B/R} (You may cast this spell for its bleed cost if an opponent has lost life this turn.)
Haste
When Opportunistic Raiders enters, if it's your main phase, after this phase, there is an additional combat phase followed by an additional main phase.

Whenever Optimistic Chronicler deals damage to a player or noncreature permanent, surveil 1, then draw a card.

Chronicle (As this enters and after your draw step, add a lore counter. Stop after III.)
[Lore I-II] You may discard a card. If you do, draw a card.
[Lore III] Create a token that's a copy of another target creature you control, except it has haste. Sacrifice it at the beginning of the next end step.

Oreskos Warden has lifelink as long as you control an enchantment.

Whenever you cast a black spell, choose up to X, where X is the number of black mana symbols in that spell's mana cost —
• Target player loses 1 life and you gain 1 life.
• Put a -1/-1 counter on target creature.
• You draw a card and you lose 3 life.
• Exile target card in a graveyard.

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

Vigilance
You may look at the top card of your library any time.
Whenever you cast a creature spell from your hand, you may cast the top card of your library without paying its mana cost if it's a creature card with lesser mana value.

Whenever Orria Mirrorpelt becomes the target of a spell, exile that spell. If Orria Mirrorpelt was its only target, copy it for each other creature target player controls. Each copy targets a different one of those creatures.

Primal {1}{G}{G} (During your turn, you may cast this spell for {1}{G}{G} instead of playing a land.)
When Orria's Ward enters, if you played a land this turn, draw a card. Otherwise, put two +1/+1 counters on it.

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

All creatures able to block Osheen Courtier do so.
Persist (When this creature dies, if it had no -1/-1 counters on it, return it to the battlefield under its owner's control with a -1/-1 counter on it.)

Flying
Whenever an opponent casts a spell, counter it unless that player pays {1} for each spell cast before it this turn.