msem-instigator

Flash
Whenever you cast an artifact or enchantment spell, put a +1/+1 counter on Observer of Secret Paths. It can't be blocked this turn.

Tribute 2 (As this creature enters, an opponent of your choice may put two +1/+1 counters on it.)
When Observing Guardian enters, if tribute wasn't paid, create a 2/2 white Cat creature token.

Other creatures you control get +2/+0.
Creatures your opponents control get -1/-1.
Fleeting {2}{B} (You may cast this spell for its fleeting cost. If you do, sacrifice it at the beginning of the next end step.)

Flying
When Obsidian Hellkite dies, it deals 2 damage to up to one target creature or planeswalker. If that creature or planeswalker would die this turn, exile it instead.
Foreshadow {4}{R}{R} (You may cast this spell for its foreshadow cost. If you do, also trigger its “dies” abilities when it enters.)

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.

Flying
Whenever you cast a noncreature spell, you may exchange control of it and target creature. Then the spell's controller may choose new targets for it.

When Occult Mastermind enters, create two 2/2 black Zombie creature tokens. When Occult Mastermind is no longer on the battlefield, sacrifice two Zombies.

At the beginning of your end step, create a token that's a copy of target transformed permanent you control, except it has “This permanent can't transform.”

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.

Slipstream (This creature can't be blocked as long as only creatures with slipstream are attacking.)
{T}: Salvage a creature card. (Exile one 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.)

Motley — At the beginning of combat on your turn, if there are five or more names among nonland permanents you control, Ochre Knight gets +2/+1 until end of turn and you gain 1 life.

Whenever one or more artifacts you control enter, put a +1/+1 counter on Octopus Shiny-Grabber and you gain 1 life.

Flash
When Odaya Magistrate enters, you may counter target noncreature spell.

Deception {2}{U} ({2}{U}, Return an unblocked attacker you control to hand: Put this card onto the battlefield from your hand tapped and attacking.)
Whenever a creature you control deals combat damage to an opponent, you may draw a card.

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.

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.

{T}: You may put a land card from your hand onto the battlefield.
{1}, {T}: Aetherize target land you control. (To aetherize, create a token that's a copy of it, then exile that token.)

First strike
Whenever Ogura, Contracted Rebel attacks, you may create a token that's a copy of another attacking creature with power less than or equal to Ogura's. Sacrifice the token at the beginning of your next end step.
{R}: Ogura gets +1/+0 until end of turn.

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.

Okrophore's power and toughness are each equal to the number of colorless cards in your graveyard.

Enchant planeswalker
Enchanted planeswalker has “+2: Draw two cards, then put a card from your hand on the bottom of your library.”

Enchant creature or planeswalker
You control enchanted permanent.
Permanents you control but don't own have hexproof.

Archive 4 (As this enters, shuffle the top four cards of your main library into a new face up library. Sacrifice this after the archived library is empty.)
Whenever you draw a card from the archived library, gain control of target nonland permanent with a lesser mana value than that card until you draw a card from the archived library.