msem-instigator

Slipstream (This creature can't be blocked as long as only creatures with slipstream are attacking.)
Concord — Once each turn, when an opponent casts a spell or activates an ability that targets a Merfolk or enchantment you control, you may counter that spell or ability if there are four or more card types among permanents you control.

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.

When Old Salt enters, forebode. (Create a tapped colorless Omen artifact token with “{T}, Discard a card, Sacrifice this artifact: Draw a card.”)

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.

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
Flying

{3}{U}, Exile a card from your graveyard: Omen Serpent can't be blocked this turn.

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.

Whenever Ontarachie Hunter attacks, each opponent loses 1 life.
At the beginning of your end step, if an opponent has 10 or less life, transform Ontarachie Hunter.

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.)

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

As an additional cost to cast this spell, you may tap any number of untapped tokens you control. This spell costs {1} less to cast for each token tapped this way.
Whenever Organic Dreadnought enters or attacks, tap target permanent an opponent controls. That permanent doesn't untap during its controller's next untap step.

Trample
Orodu, the Onyx Wave can't block.
Whenever Orodu attacks, defending player may sacrifice a creature. If they do, prevent all combat damage Orodu would deal this turn.

Pressure—{4}{U}{B}, Sacrifice two creatures. (You may cast this spell for its pressure cost if seven or more cards are in your graveyard.)
Whenever Oryxyth enters or attacks, create an X/X black Horror creature token, where X is the number of cards in your graveyard.
{U}{B}, Sacrifice another creature: Gain control of target creature with power less than the sacrificed creature's power.

Flash
Oscanthor, Trader's Ruin can't be blocked.
Whenever a creature you control deals combat damage to a player, draw a card.

{8}: Create eight 1/1 blue Tentacle creature tokens.
{8}: Counter target spell unless its controller pays {8}.
{8}: Draw eight cards.

({T}: Add {U}.)
Wanderlust — Otherworldly Island enters tapped unless you control four or more other differently named lands.
{1}, {T}: Add one mana of any color.

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.)

Concord — When Palace Servant enters, if there are four or more card types among permanents you control, create two 1/1 white Merfolk creature tokens.

Pelagic Hydra enters with seven +1/+1 counters on it.
{1}{G}: Put a +1/+1 counter on Pelagic Hydra.
Remove a +1/+1 counter from Pelagic Hydra: It deals 1 damage to target creature blocking or blocked by it.

When Pensive Muse enters, create a white Aura enchantment token named Seal's Love attached to target creature you control. The token has enchant creature and “Enchanted creatures you control get +1/+1.”

Whenever a noncreature permanent enters, Petaldiver deals 1 damage to that permanent's controller.

{U}: Pilgrim's Muse can't be blocked this turn.
{1}{U}: Return Pilgrim's Muse to its owner's hand.
{2}{U}: Scry 2.
{3}{U}: Draw a card.

Whenever you cast a red spell, target creature gets +1/+0 and gains haste until end of turn.

First strike, haste
Whenever you cast an enchantment, instant, or sorcery spell, put a +1/+1 counter on Plasma Blader.
{2}{R}{R}, Sacrifice Plasma Blader: It deals damage equal to its power to any target.