Re: samsung: while they don't make the operating system, they do bundle apps and sometimes even their own filesystem (Galaxy S1). This particular filesystem made android slow.
You can argue that this means Samsung does indeed make their own operating system.
The term OS, as it is used by media and most people today, is the software that is installed on the device before you install any additional apps to it. E.g the "App Store" is considered a part of iOS, the Internet Explorer as part of Windows, and the Homescreen and Dock of Max OSX is considered a part of OS X.
Since a Galaxy and a HTC One ship with different Applications, a different user interface and other key software components (you mentioned the filesystem on the Galaxy S), Android is less an operating system but but the cross platform foundation that most operating systems are based on today.
