If a card with no mana cost gains flashback, it has no flashback cost. It can't be cast this way.

If a card with flashback is put into your graveyard during your turn, you can cast it if it's legal to do so before any other player can take any actions.

If you cast an instant or sorcery with {X} in its mana cost this way, you still choose the value of X as part of casting the spell and pay that cost.

If you cast a spell with flashback, you can't pay any alternative costs such as overload costs. You can pay additional costs such as kicker costs. If the spell has any mandatory additional costs, you must pay those to cast the spell with flashback.

If a card has multiple instances of flashback, you may choose any of its flashback costs to pay.

If a split card gains flashback, you pay only the cost of the half you're casting.

"Flashback [cost]" means "You may cast this card from your graveyard by paying [cost] rather than paying its mana cost" and "If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack."

You must still follow any timing restrictions and permissions, including those based on the card's type. For instance, you can cast a sorcery using flashback only when you could normally cast a sorcery.

Otaria : règles, fonctionnement et interactions (MTG)

Otaria is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Instant and sorcery cards in graveyards have flashback. The flashback cost is equal to the card's mana cost. (Its owner may cast the card from their graveyard for its mana cost. Then they exile it.) Whenever chaos ensues, take an extra turn after this one.

How it works : Otaria

Original English text — Wizards of the Coast / Scryfall

#1March 19, 2021· Wizards of the Coast
If a card with no mana cost gains flashback, it has no flashback cost. It can't be cast this way.
No other card is named in this ruling.
#2March 19, 2021· Wizards of the Coast
If a card with flashback is put into your graveyard during your turn, you can cast it if it's legal to do so before any other player can take any actions.
No other card is named in this ruling.
#3March 19, 2021· Wizards of the Coast
If you cast an instant or sorcery with in its mana cost this way, you still choose the value of X as part of casting the spell and pay that cost.
No other card is named in this ruling.
#4March 19, 2021· Wizards of the Coast
If you cast a spell with flashback, you can't pay any alternative costs such as overload costs. You can pay additional costs such as kicker costs. If the spell has any mandatory additional costs, you must pay those to cast the spell with flashback.
No other card is named in this ruling.
#5March 19, 2021· Wizards of the Coast
If a card has multiple instances of flashback, you may choose any of its flashback costs to pay.
No other card is named in this ruling.
#6March 19, 2021· Wizards of the Coast
If a split card gains flashback, you pay only the cost of the half you're casting.
No other card is named in this ruling.
#7March 19, 2021· Wizards of the Coast
"Flashback [cost]" means "You may cast this card from your graveyard by paying [cost] rather than paying its mana cost" and "If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack."
No other card is named in this ruling.
#8March 19, 2021· Wizards of the Coast
You must still follow any timing restrictions and permissions, including those based on the card's type. For instance, you can cast a sorcery using flashback only when you could normally cast a sorcery.
No other card is named in this ruling.
#9March 19, 2021· Wizards of the Coast
To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.
No other card is named in this ruling.
#10March 19, 2021· Wizards of the Coast
A spell cast using flashback will always be exiled afterward, whether it resolves, is countered, or leaves the stack in some other way.
No other card is named in this ruling.
#11March 19, 2021· Wizards of the Coast
You can cast a spell using flashback even if it was somehow put into your graveyard without having been cast.
No other card is named in this ruling.
#12October 1, 2009· Wizards of the Coast
A face-up plane card that's turned face down becomes a new object with no relation to its previous existence. In particular, it loses all counters it may have had.
No other card is named in this ruling.
#13October 1, 2009· Wizards of the Coast
The controller of a face-up plane card is the player designated as the "planar controller." Normally, the planar controller is whoever the active player is. However, if the current planar controller would leave the game, instead the next player in turn order that wouldn't leave the game becomes the planar controller, then the old planar controller leaves the game. The new planar controller retains that designation until they leave the game or a different player becomes the active player, whichever comes first.
No other card is named in this ruling.
#14October 1, 2009· Wizards of the Coast
If an ability of a plane refers to "you," it's referring to whoever the plane's controller is at the time, not to the player that started the game with that plane card in their deck. Many abilities of plane cards affect all players, while many others affect only the planar controller, so read each ability carefully.
No other card is named in this ruling.
#15October 1, 2009· Wizards of the Coast
If you roll multiple times in the same turn, you'll take that many extra turns after this one.
No other card is named in this ruling.
#16October 1, 2009· Wizards of the Coast
A plane card is treated as if its text box included "When you roll , put this card on the bottom of its owner's planar deck face down, then move the top card of your planar deck off that planar deck and turn it face up." This is called the "planeswalking ability."
No other card is named in this ruling.