msem-instigator

Exile target permanent, then return it to the battlefield under its owner's control. If that permanent was an attacking creature, you may have it enter tapped and attacking.

Menace
Whenever you cast a glitched spell, Null Entity gets +2/+0 until end of turn for each time that spell was glitched.

Activated abilities of other permanents cost an additional “Sacrifice a permanent” to activate unless they're mana abilities.
{2}, {T}, Sacrifice Nullifier of Planar Energy: Remove all counters from target nonland permanent.

Flying
Whenever you cast a creature spell for the first time each turn, create a 1/1 green Insect creature token.

Flying, deathtouch

Menace
When OM-6 Walker dies, OM-6 Walker deals damage to each player equal to twice the number of nonbasic lands that player controls.

Flash
Enchant creature
Enchanted creature gets +2/+2.

Reach
When Oaken Sentry enters, you gain 1 life for each creature card in your graveyard.

Whenever a creature enters, destroy it unless its controller pays 3 life.

When Obscure Apostle 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.”)
Whenever you sacrifice an Idol, Obscure Apostle can't be blocked this turn.

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.

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.

You may pay {R} and tap an untapped Warrior you control rather than pay this spell's mana cost.
Choose one —
• Double the power of target creature you control until end of turn.
• Target creature you control deals damage equal to its power to target creature or planeswalker.

Whenever you cast a spell, note its colors.
At the beginning of combat on your turn, up to X target creatures get +2/+0 and gain menace until end of turn, where X is the number of colors noted with Off-Set Rumormonger.

When Offer the Heart enters, draw a card, then choose a card name.
Whenever a creature with the chosen name dies, you gain 1 life and may draw a card.

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.

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

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.

Cadence (Whenever you cast a spell, if its mana value is greater than this creature's power, put a +1/+1 counter on this creature.)
Creatures with power less than Olten Survivalist's power can't block creatures you control.

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.

Target creature gets -5/-5 until end of turn. Scry 1.

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.

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

Equipped creature gets +0/+3 and has menace. (It can't be blocked except by two or more creatures.)
Equip {1}

Post a bounty on target creature you don't control, then each opponent sacrifices a creature. (Put a bounty counter on that creature. It has “When this creature dies, each opponent draws a card” as long as it has a bounty counter on it.)