Doomskar Oracle

• WOTC Official Reminder Textofficial rules
  Foretell — Foretell [cost] (During your turn, you may pay and exile this card from your hand face down. Cast it on a later turn for its foretell cost.)

• WWizards of the Coast2021-02-05
  Official Ruling
  The triggered ability can trigger only once each turn. The ability will resolve before the second spell does. It doesn’t matter if the first spell you cast that turn has resolved, was countered, or is still on the stack.
• WWizards of the Coast2021-02-05
  Official Ruling
  The triggered ability triggers only if the creature that has it is on the battlefield as you cast your second spell. Spells you cast in a turn before that creature entered the battlefield will count. In other words, the ability won’t trigger if the creature with the ability is the second spell you cast during a turn or if you have already cast two or more spells by the time that creature enters the battlefield that turn.
• WWizards of the Coast2021-02-05
  Official Ruling
  Because exiling a card with foretell from your hand is a special action, you can do so any time you have priority during your turn, including in response to spells and abilities. Once you announce you’re taking the action, no other player can respond by trying to remove the card from your hand.
• WWizards of the Coast2021-02-05
  Official Ruling
  If you’re casting a foretold card from exile for its foretell cost, you can’t choose to cast it for any other alternative costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, those must be paid to cast the spell.
• WWizards of the Coast2021-02-05
  Official Ruling
  Casting a foretold card from exile follows the timing rules for that card. If you foretell an instant card, you can cast it as soon as the next player’s turn. In most cases, if you foretell a card that isn’t an instant (or doesn’t have flash), you’ll have to wait until your next turn to cast it.

Rulings via Scryfall