Unlike a standard action that can only be taken during the character's initiative turn or a resistance action that can only be used in defence, opportunity actions can only be performed during another player's initiative turn.
These actions require the use of a card with the Opportunity keyword combined with any available and appropriate cards. They are resolved exactly the same way as initiative actions.
Each opportunity card specifies a trigger event that enables its use. For example, the Retreat card can only be used when a foe moves towards the character. Unless the circumstances fit the trigger, an opportunity action cannot be used. The character must be aware of the trigger, so for example the Retreat card cannot be used if the character doesn't notice the foe approaching.
The character is limited to the type of action described by the card. A Retreat card cannot be used to attack a foe and a Reflex Attack card cannot be used to run away.
It is up to the player with the opportunity card to ensure they pay attention and declare their opportunity action at the right time. If the player whose initiative turn it is has already played another action, the opportunity has been lost.
Each player may only make one opportunity action per action by the character whose initiative turn it is. For example, character A is in their initiative turn and makes a movement action. Character B takes an opportunity action but cannot take another opportunity action until character A acts again.
An action may trigger opportunity actions from two or more players. In these circumstances, the one who declared first gets to act first but all the characters who declared must act.
Opportunity cards can only be triggered by standard actions. Other players' opportunity actions, resistance actions or any other types of action do not trigger opportunities.