If a spell or ability an opponent controls instructs you to perform an action unless you sacrifice a permanent (as Ogre Marauder does), you can’t choose to sacrifice a permanent. You must perform the action. On the other hand, if a spell or ability an opponent controls instructs you to sacrifice a permanent unless you perform an action (as Rishadan Brigand does), you can choose whether or not to perform the action. If you don’t perform the action, nothing happens, since you can’t sacrifice any permanents.