msem-instigator

Germinate, then creatures you control that are Saprolings or have counters on them gain indestructible until end of turn. (To germinate, put a +1/+1 counter on a creature or create a 1/1 green Saproling creature token.)

When Ontarachie Envoy enters, nourish, then germinate. (To nourish, put a +1/+1 counter on this creature or create a 1/1 white Citizen creature token. To germinate, put a +1/+1 counter on a creature you control or create a 1/1 green Saproling creature token.)

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

Destroy target artifact. Scry 1.

Trample
Whenever Openmaw Demon or another Azra or Demon you control enters, you may exile target card from a graveyard.

Opera House enters tapped.
When Opera House enters, add one mana of any color.
{T}, Sacrifice Opera House: Search your library for a basic land card, put it onto the battlefield tapped, then shuffle.

Converge — When Operations Manager enters, create X 1/1 colorless Servo artifact creature tokens, where X is the number of colors of mana spent to cast Operations Manager.

Lifelink (Damage dealt by this creature also causes you to gain that much life.)
When Ophelia's Neophyte enters, you may have another target creature you control gain lifelink until end of turn.

When Ophorio Scavenger enters, create an Idol token. (It's an artifact with “{1}, Sacrifice this artifact: Put a +1/+1 counter on target creature. Activate only as a sorcery.”)
Ophorio Scavenger gets +1/+1 as long as you control three or more artifacts.

Enchant creature
Enchanted creature can't attack or block unless its controller pays {3}.
Activated abilities of enchanted creature cost {3} more to activate.

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

Draw three cards, then discard two cards.

When Oracle Crab enters, scry 2. (To scry 2, look at the top two cards of your library, then put any number of them on the bottom of your library and the rest on top in any order.)

When Oracle of Edicts enters, adjudicate. (Create a white Aura enchantment token with enchant creature and “Sacrifice this token: Untap enchanted creature” attached to target creature.)
Whenever an Aura you control enters, scry 1.

Whenever Oracle of Flames becomes tapped, up to one target creature you control gets +2/-2 until end of turn.

Trample, reach
When Orchard Overseer dies, surveil 2.
Foreshadow {5}{G} (You may cast this spell for its foreshadow cost. If you do, also trigger its “dies” abilities when it enters.)

Target creature gets -5/-5 until end of turn.
Judge up to one target creature you control. (Honor it by putting a +1/+1 counter on it or condemn it by sacrificing it to draw a card.)

Identity — When Order's Anointer enters, if you control at least two other creatures that share a creature type with it, creatures you control get +1/+1 until end of turn.

Whenever Ore Tunneler deals damage to an opponent, tap target land they control. It doesn't untap during its controller's next untap step.

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

Whenever Organ Scavenger perseveres, each opponent loses 2 life. (A creature perseveres when it survives damage.)

Destroy target artifact or enchantment with mana value 3 or less.
Ferocious — You gain 4 life if you control a creature with power 4 or greater.

Equipped creature gets +1/+1.
Whenever equipped creature deals combat damage to a player, Ornamental Longbow deals 1 damage to any target.
Equip {2}

{1}, Sacrifice Ornate Incense: Draw a card. You gain 1 life.

{2}{W}: Ornate Legionnaire gains first strike until end of turn.