DOS/65 is a CP/M like operating system for the Commodore 64.  Inside the archive is the program with full source and also a PDF of the website (http://www.z80.eu/dos65.html).  Use the website PDF to understand the correlation of the files.

Supra 64 v1.1 released in 1990 - Ran across this complete GUI operating system for the Commodore 64 that looks similar to GEOS but doesn't appear to be a knock-off.  There are several programs included such as a spreadsheet and wordprocessor.  There is also a custom BASIC programming language included called Supratec.  Full docs are included in the archive.

Triangular uOS 1.25 for the Commodore 64, by Michael Goral, Released August, 2022.  Included in the archive is the operating system, full documentation and a complete change log.  See the extended description to see what's new in version 1.25

It's really amazing what has come out for our humble Commodore 64 over the years, and this piece of software is a good example.  Written entirely in BASIC, this is a complete OS!!!  It even has applications integrated.  I strongly recommend you check this out.  Here are some links to additional information / resources:

Lemon64 forum thread can be found here.

YouTube video can be found here.

Triangular uOS v1.25 Software Development Kit by Michael Goral (Released November, 2022) - This archive includes the Programmers Reference Guide, a memory map spreadsheet,  and the system variables spreadsheet.  Everything you would need to start developing for Triangular u)S is included. 

Here's a link to the Triangular uOS v1.25 released in August, 2022.

Here's a link to the Triangular uOS v1.25 User's Manual