OPERATING SYSTEM. The operating system is the lowest level software that enables you to interact with the computer and which controls the basic functioning of the machine as far as storage, communications and task management are concerned. The O.S. is the most important program running on a computer, as without one it would be impossible to use. Examples of well-known operating systems are Windows 95/98/2000/NT, Linux, MS/DOS, UNIX, OS/2, Mac OS, etc.