grammatical number - Plural of an initialism that ends with the letter S

I was answering something on Super User and wrote OSes as part of my normal flow without really thinking about it. On a re-read I decided that it didn't look right, so I changed it to OSs, which still felt incorrect. I also considered OS's, but that didn't feel right either, so I thought I'd ask on here.

In this specific case, what is the written plural form for OS?
The intention is to mean multiple Operating Systems.

And, is there a general rule for the plural form of an initialism that ends with the letter S?

Answer

A search on Google for OSes returns results from several established websites such as infoworld.com, osnews.com and linux.com, which suggests OSes is the accepted form.