msem-instigator

Flying
When Novaheart Guardian enters, it deals 4 damage to any target.
Whenever a creature or planeswalker dies, put a +1/+1 counter on Novaheart Guardian.

Reverberate—Sacrifice a Swamp. (When you cast this spell, copy it if you paid its reverberate cost.)
All creatures get -2/-2 until end of turn.

Trample
Whenever you attack with another creature with power 4 or greater, put a +1/+1 counter on OH-14 Diamond.

Trample
Assault {1}{G} (Whenever this attacks, if it has no +1/+1 counters on it, you may pay its assault cost to put a +1/+1 counter on it.)
Whenever a creature you control with a +1/+1 counter on it deals combat damage to a player, draw a card.

Oathbreaker Orc attacks each combat if able.
Whenever Oathbreaker Orc attacks, it deals 2 damage to defending player unless that player sacrifices a permanent.

Obliterating Blast deals 3 damage to any target. A creature dealt damage this way this turn can't be regenerated this turn. If that creature would die this turn, exile it instead.

Obscoriate deals 4 damage to target creature or planeswalker. The next time that permanent would die this turn, instead put a basalt counter on it, it becomes a Mountain land named Charred Remains, loses all other card types, and gains “At the beginning of your upkeep, Charred Remains deals 1 damage to you.” (It has “{T}: Add {R}” and loses abilities it already had.)

This spell costs {1} less to cast for each artifact and/or enchantment you control.
Trample
{1}, Sacrifice another cryptic permanent you control or exile a cryptic card from your graveyard: Put a +1/+1 counter on Observer of Aeons. (Artifacts, enchantments, and modal spells are cryptic.)

Whenever you attack, you may double target attacking creature's power. If you do, sacrifice it at the beginning of the next step.
Whenever one or more red creatures you control enter, creatures you control get +1/+0 and gain trample until end of turn.

As Ocean Monastery enters, you may pay 2 life. If you don't, Ocean Monastery enters tapped.
{T}, Sacrifice Ocean Monastery: Search your library for a Plains or Island card, put it onto the battlefield, then shuffle.

Flash
Ocean's Edge Kraken enters with a hexproof counter on it.
{6}{U}{U}, Remove a hexproof counter from Ocean's Edge Kraken: Return all other creatures to their owners' hands.

Exile up to two target creatures controlled by different players. Return those creatures to the battlefield under their owners' control at the beginning of the next end step.

Each player sacrifices a creature. If a player lost 3 or more life this turn, each opponent sacrifices a creature instead.

Trample, haste
When Oil Driller enters, salvage two cards at random. (Exile those cards from your graveyard until this creature leaves the battlefield. Whenever this creature deals combat damage to a player, put a card it salvaged into your hand.)
At the beginning of your end step, sacrifice Oil Driller.

Other Merfolk you control get +1/+1.
When Olantin Pearlreader enters, scry 2. Then draw a card if you control two or more other Merfolk.

Target creature gets -X/-X until end of turn, where X is the number of enchantments you control.

Oldgrove Nurturer enters with a +1/+1 counter on it.
{G}, Remove a +1/+1 counter from a creature you control: Create a 1/1 colorless Construct artifact creature token.

Whenever you attack with one or more Pirates, create that many 1/1 white Bird creature tokens with flying that are tapped and attacking. At end of combat, choose one of those tokens, then exile the rest.

When Olten Surveyor enters, choose any number. Each opponent creates a 1/1 colorless Servo artifact creature token for each mode chosen.
• Destroy target artifact or enchantment with mana value 3 or less.
• Shuffle up to four cards from target player's graveyard into their library.
• You gain 4 life.

Trample
Ward {3} (Whenever this creature becomes the target of a spell or ability an opponent controls, counter it unless that player pays {3}.)
Cycling {2} ({2}, Discard this card: Draw a card.)

Look at the top two cards of your library. Put one of them into your hand and the rest into your graveyard.
Rerun (If you cast this from your hand, exile it as it resolves. Whenever a creature you control dies, you may cast this card from exile without paying its mana cost.)

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

This spell costs {1} less to cast for each card in your graveyard.
Menace

This spell costs {G} less to cast if it targets a blue or black permanent.
Destroy target artifact, enchantment, or planeswalker.

This spell costs {1}{B} less to cast if it targets a green or white permanent.
Destroy target creature or planeswalker.