OS/360 was a victim of over-ambition. Originally, it was to run on all models of System/360 (maybe not the Model 20, which implemented only a subset of the ISA), and it was complex enough that there were challenges in building it. IBM ended up spinning off the smaller models to IBM Germany, DOS/360 came from there, and subsetting OS/360 into 3 levels, so they could get something out the door only a bit late. OS/360 by some measures is one of the most successful operating systems ever: IBM's current z/OS is a remote descendent of it. Yes, it is pretty horrible to use (three letters: JCL), but it certainly was successful.

“The Mythical Man-Month” is not about OS/360 as such, but about project planning and specifically what was learned about project management during the development.

Sadly we went from over-ambition to mostly UNIX clones in modern OSes, which triggered Rob Pike's famous rant on systems programming, or his remarks on Slashdot interview.