Buying a refurbished Dell with no Operating System????

Hi all you computer wizards.  I have another question.  A friend  (yes really a friend) is considering buying a refurbished Dell computer Omniplex  (?) Optiplex (?)but there is no "Operating System"  included.   Is this  a really bad idea?  I am not sure what the "OS" is but I thought it was the Windows application that only needs to be downloaded into the computer.  How far off am I on this answer?  THis person only needs a computer for school work, and taking an online class.  Not any career that is based on a computer system. 

As usual, your advice is so greatly appreciated!  Thanks!

~Jackie