What's the plural of OS?

By virtue of my ever-popular Effective Writing training course, and despite no formal education in English above GCSE level, I am somewhat known for being a style and grammar authority / pedant. This means two things: first, my mother is very proud; and second, I get the occasional request to arbitrate on particular language usage.
This week: What's the plural of OS (where OS is the common abbreviation for Operating System)? Is it OSs? OSes? OS's?
I didn't know the answer offhand, so I looked it up in four different style guides and drew a blank. However, there are some good rules of thumb that might apply in this case.
One rule is: treat the abbreviation as though it were the expanded form. That would tend to favour "OSs", since we wouldn't add -es to the word System. However, this rule has some notable, and bitterly contested, exceptions, both in written and spoken English. For example, GIF is correctly pronounced jif, even though the initial G stands for Graphics.
Another rule is: be guided by pronunciation. This approach is very helpful to the reader as well as the author. I find this particularly helpful when trying to ascertain where to place an apostrophe in a word that naturally ends in an S. Is it pronounced St James' Place or St James's Place? This rule would favour "OSes" as the correct plural.
But there's one overriding rule in language, which is that common usage determines the rules. Dictionaries don't set or enforce language rules; they adapt to reflect the way the language is actually used. Even where a word or punctuation mark is technically wrong, the consensus of actual usage overrides it every time. The word "literally" is a very obvious case in point, having come to mean the exact opposite of its original definition. Language evolves, and every pedant in the world is powerless to stop it.
On these grounds, it appears that OSs is likely the more common, and therefore correct, usage. This article cites the Microsoft style guide and a mostly-respectful debate on the issue as evidence for this.
My final, and overarching, rule in a situation like this one is: as long as it's not obviously incorrect, choose an answer for a good reason, and then stick to it consistently. If a grammar pedant believes that "OSs" is inherently wrong, then you aren't ever going to change their mind; just as you aren't ever going to solve the war over spaces after a full stop.
Many thanks to my colleague Jack for raising this unexpectedly rather interesting question.
A version of this article was originally published on my company internal blog.