OSX has native smart card support When you have opensc installed it will detect your yubikey as a smartcard and ask if you want to unlock your computer with it

OSX has native smart card support When you have opensc installed it will detect your yubikey as a smartcard and ask if you want to unlock your computer with it :edit: you also need to go to System Preferences > Security & Privacy > Advanced > Turn on screen saver when login token is removed