Good post Ennis, keep them coming... In addition, I found this in "Operating Systems Concepts" by S. Galvin, it helped me defining an O.S.

An operating system is similar to a government. The components of a computer system are its hardware, software, and data. The operating system provides the means for the proper use of these resources in the operation of the computer system. Like a government, the operating system performs no useful function by itself. It simply provides an environment within which other programs can do useful work.

Operating System Concepts, 5th edition, by Silberschatz Galvin
John Wiley & Sons, 1999
ISBN 0-471-36414-2