What is the plural of OS?

[dclamp]

What do you guys think the plural way to write/say the acronym OS (short of operating system of course) is correct in English?

I have looked around and found some people saying the plural is:

OS
OSes
OS's
OSs

I personally always say it as if it was spelled like OSes or OS's, but I don't have to write it too often...

Based on a site showing the rules of when an apostrophe is used, it claims:
1) to form possessives of nouns
2) to show the omission of letters
3) to indicate certain plurals of lowercase letters.

But I don't really think any of those cases apply here...

OI is actually the correct term.

Examples:
OS -> OI
Octopus -> Octopi
Cactus -> Cacti