Platform vs operating system

I would say Windows in a operating system while a Mac is a platform.

An OS is pure software while a platform is the combination between the OS and the kind of hardware, especially the CPU, it runs on.

It just depends on what context you are talking:

If you say the Windows platform to a bunch of developers, you could mean the Windows development platform (vs. the OSX platform or the Linux platform), if you said Windows OS instead, that could also mean just as many things.

If you were talking to hardware people and said the Windows platform, they might 'assume' you meant the x86 architecture (since Windows is deployed on more x86/64 platforms than other types), but would probably ask what you meant, but if you mentiond the 'old Mac platform' to a bunch of hardware people you might spark talks about the PPC (PowerPC) processor.

The specific word 'platform' is used interchanably to mean a lot of differnet things within the context of the CS/IT world (unfortunly this is the case with many many other IT/CS lexicon), again it really depends on the context of what you're saying.