There are many open-source OS distributions (distros) branching out from the Linux Kernel. Ubuntu, Linux Mint, Puppy, to name but three.
pervasive computing
OS is an open-source operating system
LINUX and Unix OS is open-source operating system.....! but Windows version OS is not the open-source operating system....!
Yes it is!
Classic Mac OS is closed source. Mac OS X is closed source as well, but many libraries and applications within it are open source, e.g. XNU the kernel is open source, and Darwin the underlying UNIX operating system was open source.
Having the source code open for anyone to edit or modify for free.
penis
GNU/Linux(same thing) There's: Red hat, Ubuntu, LFS(Linux from scratch) and many others
The Darwin kernel of Mac OS X is open source, as are many other parts of the OS. (See links below) but Mac OS X itself is not open source and Apple only allows it to run on Apple hardware.
Is MacOS an open source system? No.
Meaning of open source OS
Android has been an open source OS since 2008.