There are two options for obtaining a copy of KOS for your use.

Option #1 is to download a release archive. These are snapshots made of the working tree once in a while and have an associated version number in the same style as Linux kernel releases (e.g. 1.2.1).

Note that there may be long breaks between releases. Version 2.0.0 was released on May 23, 2013, and the previous version (1.2.0) was released exactly ten years earlier. The release versions may end up being quite old and don't always really reflect the current state of KOS well at all. The Git versions of KOS are generally pretty stable and don't really tend to throw away the old APIs as the paragraph up above may suggest.

Release archives can be obtained here.

Option #2 is to follow the source control. This is bleeding edge stuff, but it's generally recommended anyway because KOS itself is pretty stable. If you want to follow the devel version (e.g., 2.0.x) then you need to be on this version. If you make local modifications to your tree then you probably also want to be on here so you can merge new updates. Note that it's also possible to follow a branch, for example the 1.2.x branch. This lets you track the "stable" versions in a simpler way.

KOS is now developed (as of early June 2012) using Git. For more info on getting setup with that, go here.