This is among the most important parameters in choosing the tablet computer.
iOS - the tablet computers manufactured by Apple are equipped with the iOS operating system that is very convenient and high-speed, however, this is a closed system where no special configurations and extensions are possible. However, within the iOS currently most applications (apps) are available. iOS is compatible with Apple cloud services for which a fee is charged in most cases.
Android is a free of charge operating system developed by Google, similar to the iOS, however with all the possibilities to make a lot of changes therein. Besides, the Android offers good compatibility with the Google free cloud services - Gmail, Picasa, Youtube, Maps, Google+, Contacts, Calendar etc. The number of the Andoid users is growing most rapidly, therefore, this is a very perspective operating system.
Windows - the classic Microsoft Windows system that allows to use the convenient .EXE applications in the tablet computer. It has to be admitted that currently they are better suited to desktop computers, therefore you should choose this operating system only in cases when there is a Windows application without which you cannot do.
Other (Bada, MeeGo, WebOS, BrewMP, Symbian) - currently on the market of the mobile operating systems also several exotic operating systems are available besides the above listed, still, you should take into account that the biggest fight over this market share is over, therefore you should take care if you choose an operating system which is not iOS, Windows or Android, because, contrary to the classic computers, in most tablet computers it is not possible to change the operating system after buying the tablet.