The Web pages, videos, and blog entries found on this Website are meant to teach visitors about several different applications that work equally well on Windows, Mac OS, and Linux desktop operating systems. They are known as cross-platform applications. Examples of these applications are LibreOffice and OnlyOffice, two office suites. One of the things "OS" in OS-College stands for is office suites. The Website also has videos, pages, and articles about Google's office applications and Microsoft Office Online.
Besides office suites, there are videos and written instructions and introductions to Gimp, OpenShot, and other software that works equally well on the operating systems mentioned in the previous paragraph. "OS" stands for other software.
Another thing "OS" means is operating systems. Videos, Web pages, and articles here cover several different Linux desktop operating systems. The main one focused on is Linux Mint with the Cinnamon interface, but information about Ubuntu and Manjaro with KDE Plasma also can be viewed here.