msem-instigator

When Ontarachie Emissary leaves the battlefield, you may search your library for a basic land card, put it onto the battlefield tapped, then shuffle.

Deathtouch
Whenever Oozing Adder attacks, you may have target creature defending player controls block it this combat if able.

Opalescent Heirloom enters with a charge counter on it for each color of mana spent to cast it.
Equipped creature gets +1/+1 for each charge counter on Opalescent Heirloom.
Equip—{3} or remove a charge counter from Opalescent Heirloom.

{T}: Add {U} or {R}. You lose 1 life unless you have less life than an opponent.

Return target creature card from your graveyard to the battlefield, then rediscover a creature card. (Look at the top card of your library. You may reveal it and put it into your hand if it's a creature card. Otherwise, put it back or into your graveyard.)

At the beginning of your upkeep, reveal the bottom card of your library. You may cast it if it's a creature card. Otherwise, exile it.

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.

Creatures can't attack you unless their controller pays {3} for each of those creatures.

Whenever you cast your first instant or sorcery spell each turn, create a Treasure token. (It's an artifact with “{T}, Sacrifice this artifact: Add one mana of any color.”)

When Opulent Proprietor enters, you may search your library and/or graveyard for an Aura, Aurashaper, Equipment, or Vehicle card, reveal it, and put it into your hand. If you search your library this way, shuffle.

Whenever you cast a spell during an opponent's turn, put a +1/+1 counter on Oracle of Tides.

Menace
At the beginning of your end step, if a player lost 3 or more life this turn, put a +1/+1 counter on target Azra or Demon you control.

Enchant creature
Enchanted creature gets -3/-3.
When enchanted creature dies, its controller may scry 1.

Whenever you sacrifice an Idol, put a +1/+1 counter on Orchard Botanist.

Ward—Pay 2 life.
When Orchid Suit enters, target opponent reveals their hand, then exiles a nonland card of their choice from it until Orchid Suit leaves the battlefield.

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.

Each instant and sorcery card in your hand has cycling {2}. ({2}, Discard this card: Draw a card.)
At the beginning of your end step, if there are four or more instant and/or sorcery cards in your graveyard, you may pay {1}{U/R}. If you do, return an instant or sorcery card at random from your graveyard to your hand.

Draw two cards. Create a tapped colorless artifact token named Lotus Petal with “{T}, Sacrifice this artifact: Add one mana of any color.”

Whenever Orphan Prodigy is dealt damage, it deals that much damage to target player.
Attain {4}{R} ({4}{R}: Transform this. Spend at least three colors of mana to activate this, and only as a sorcery.)

All creatures able to block Orphaned Nuckalavee do so.

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.

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.)

Exile target nontoken permanent you control, then return that card to the battlefield under its owner's control.

When Oshiruko Purifier enters, create a 2/2 colorless Spirit creature token.
Colorless creatures you control have lifelink.

Distorted (This creature can be blocked by tapped creatures but not untapped creatures.)
Oslyd Heretic gets +1/+1 as long as you control no other creatures.