You don't lose unspent green mana as steps and phases end.

Omnath, Locus of Mana gets +1/+1 for each unspent green mana you have.

Target creature gets +2/+2 until end of turn. Untap it. (Then exile this card. You may cast the creature later from exile.)

Spree (Choose one or more additional costs.)

+ — Return target creature card from your graveyard to the battlefield.

+ — Return target Mount or Vehicle card from your graveyard to the battlefield.

+ — Return target Aura or Equipment card from your graveyard to the battlefield attached to a creature you control.

Enchant creature

At the beginning of your upkeep, sacrifice a creature.

Enchanted creature gets +3/+3 and has trample.

Enchanted creature is a Demon Spirit.

Target creature gets +X/+0 until end of turn, where X is its power.

Target player draws a card.

Retrace (You may cast this card from your graveyard by discarding a land card in addition to paying its other costs.)

When you have 10 or less life, if Opal Avenger is an enchantment, Opal Avenger becomes a 3/5 Soldier creature.

When an opponent casts a creature spell, if Opal Guardian is an enchantment, Opal Guardian becomes a 3/4 Gargoyle creature with flying and protection from red.

All Slivers have "Whenever this permanent becomes the target of a spell an opponent controls, you may draw a card."

Open Fire deals 3 damage to any target.

Whenever Ophidian attacks and isn't blocked, you may draw a card. If you do, Ophidian assigns no combat damage this turn.

At the beginning of each upkeep, if you control no Snakes, create a 1/1 black Snake creature token with deathtouch.

Flash

You control your opponents while they're searching their libraries.

While an opponent is searching their library, they exile each card they find. You may play those cards for as long as they remain exiled, and you may spend mana as though it were mana of any color to cast them.

Counter target spell unless its controller pays for each card in your hand.

When Oran-Rief Ooze enters the battlefield, put a +1/+1 counter on target creature you control.

Whenever Oran-Rief Ooze attacks, put a +1/+1 counter on each attacking creature with a +1/+1 counter on it.

Permanents enter the battlefield tapped.

Orchard Spirit can't be blocked except by creatures with flying or reach.

, Sacrifice a creature: Orcish Bloodpainter deals 1 damage to any target.

: Orcish Cannoneers deals 2 damage to any target and 3 damage to you.

, Sacrifice an artifact: Orcish Mechanics deals 2 damage to any target.

When Orcish Paratroopers enters the battlefield, flip it from a height of at least one foot. Sacrifice Orcish Paratroopers unless it turns over completely at least once during the flip and lands face up.

Orcish Veteran can't block white creatures with power 2 or greater.

: Orcish Veteran gains first strike until end of turn.

, Pay 1 life: Counter target black spell.

When this creature enters the battlefield, you may exile target non-Horse creature you control that wasn't put onto the battlefield with this ability this turn, then return it to the battlefield under its owner's control.

, Exile three cards from your graveyard: Target player loses 3 life.

Whenever one or more creatures attack you, you gain 1 life for each attacking creature.

: Prevent all damage that would be dealt to target creature this turn.

Grandeur — Discard another card named Oriss, Samite Guardian: Target player can't cast spells this turn, and creatures that player controls can't attack this turn.

At the beginning of combat on your turn, put a +1/+1 counter on target creature.

Whenever Ornery Tumblewagg attacks while saddled, double the number of +1/+1 counters on target creature.

Saddle 2 (Tap any number of other creatures you control with total power 2 or more: This Mount becomes saddled until end of turn. Saddle only as a sorcery.)

At the beginning of your upkeep, each player may put two +1/+1 counters on a creature they control. If a player does, creatures that player controls can't attack you or planeswalkers you control until your next turn.

Haunt (When this creature dies, exile it haunting target creature.)

When Orzhov Euthanist enters the battlefield or the creature it haunts dies, destroy target creature that was dealt damage this turn.

: Add or .

, , Sacrifice Orzhov Locket: Draw two cards.

When Oubliette enters the battlefield, target creature phases out until Oubliette leaves the battlefield. Tap that creature as it phases in this way. (Auras and Equipment phase out with it. While permanents are phased out, they're treated as though they don't exist.)

Art rampage 2 (Whenever this creature becomes blocked by a creature, it gets +2/+2 for each creature in the blocker's art beyond the first.)

When Out of Time enters the battlefield, untap all creatures, then those creatures phase out until Out of Time leaves the battlefield. Put a time counter on Out of Time for each creature that phased out this way.

Vanishing (At the beginning of your upkeep, remove a time counter from this enchantment. When the last is removed, sacrifice it.)

When Outcaster Trailblazer enters the battlefield, add one mana of any color.

Whenever another creature with power 4 or greater enters the battlefield under your control, draw a card.

Plot (You may pay and exile this card from your hand. Cast it as a sorcery on a later turn without paying its mana cost. Plot only as a sorcery.)

Flanking (Whenever a creature without flanking blocks this creature, the blocking creature gets -1/-1 until end of turn.)

: The next 1 damage that would be dealt to Outrider en-Kor this turn is dealt to target creature you control instead.

At the beginning of combat on your turn, you may pay . If you do, put a +1/+1 counter on Overclocked Electromancer.

Whenever Overclocked Electromancer attacks, double its power until end of turn.

Whenever Overclocked Electromancer deals combat damage to a creature, if that creature was dealt excess damage this turn, you get X , where X is that excess damage.

Sacrifice a land: You gain 3 life.

Enchant land

Whenever enchanted land is tapped for mana, its controller adds an additional .

Counter target spell unless its controller pays for each artifact you control.

When Overwhelmed Archivist enters the battlefield, draw a card, then discard a card.

Disturb (You may cast this card from your graveyard transformed for its disturb cost.)

When Ovinomancer enters the battlefield, sacrifice it unless you return three basic lands you control to their owner's hand.

, Return Ovinomancer to its owner's hand: Destroy target creature. It can't be regenerated. That creature's controller creates a 0/1 green Sheep creature token.

Mama's Coming — Whenever Owlbear Cub attacks a player who controls eight or more lands, look at the top eight cards of your library. You may put a creature card from among them onto the battlefield tapped and attacking that player. Put the rest on the bottom of your library in a random order.

Attacking creatures get +X/+0 until end of turn, where X is the number of players being attacked.

Draw a card.

Convoke (Your creatures can help cast this spell. Each creature you tap while casting this spell pays for or one mana of that creature's color.)

Target creature gets +3/+3 until end of turn.

Menace

Whenever a player casts their first spell each turn, they create a Treasure token.

Whenever an artifact an opponent controls is put into a graveyard from the battlefield, Pain Distributor deals 1 damage to that player.

Whenever an opponent is dealt 3 or more damage by a single source, that player discards a card.

Converge — You draw X cards and you lose X life, where X is the number of colors of mana spent to cast this spell.

Each player may bid life. You start the bidding with a bid of any number. In turn order, each player may top the high bid. The bidding ends if the high bid stands. The high bidder loses life equal to the high bid and draws four cards.