A character can take any number of opportunity actions during a round, but no more than one during each other combatant's turn, and none during the character's own turn. Even if a character is able to take two or more different opportunity actions, the character is limited to one opportunity action of any kind per turn.
Each opportunity action has a trigger that allows its use. An opportunity action interrupts the action that triggered it. It completes prior to the interrupted action, and if the interrupted action becomes impossible to complete, it is lost.[PHB:268] Forced movement never triggers any kind of opportunity action, even if movement is listed as the action's trigger.[PHB:285]
One common opportunity action available to all characters is the opportunity attack action. It is triggered whenever another combatant provokes an opportunity attack, and allows a character to make a melee basic attack against the triggering combatant, if able.[PHB:290] Other opportunity actions include powers such as the shaman's spirit's fangs and spirit's shield.
An immediate action, like an opportunity action, can only be taken in response to a trigger during another combatant's turn. A character can only take one immediate action per turn, however, and an immediate action typically has a stronger effect than an opportunity action.