Olivia, Mobilized for War

Oracle Text

Flying

Whenever another creature enters the battlefield under your control, you may discard a card. If you do, put a +1/+1 counter on that creature, it gains haste until end of turn, and it becomes a Vampire in addition to its other types.

Card Rulings

4/8/2016 You decide whether to discard a card as the triggered ability of Olivia, Mobilized for War resolves. If you do, the rest of the ability’s effects happen before any player may take actions.
4/8/2016 If the entering creature leaves the battlefield before the triggered ability of Olivia, Mobilized for War resolves, you may still discard a card. The rest of the ability has no effect.
4/8/2016 If you discard a card, the creature only gains haste until end of turn, but the +1/+1 counter and Vampire subtype remain indefinitely.

Additional Information