Yes, an operating system is already installed on the MacBook. That's the great thing about Apple devices. Apple has always supplied the hardware and software from a single source. That's why the operating system and the most important application software for text, photo, video and music management and editing are already installed and included in the price of the device. So are updates AND upgrades of later operating system versions. Since everything comes from a single source, it usually not only works better because it is coordinated. This also puts the higher price of Apple devices into perspective - at least a little ;-)