An O.S. can be an operating system as well as this type of software like Firefox, GIMP & Thunderbird
... Key 
features: Libre Office, Firefox, Thunderbird, built-in Ubuntu Software Center, F-
spot, .... 13 best free and open source inventory management systems... [DOC] [TLE] 16 Open Source Alternatives for Your Favorite Programs - TheITBros [PAR] Feb 5, 2015 ...