I think the best choice would be OSes. Here's my argument:

I'm not convinced that being an acronym is of any signifcance here. I believe that the fact that it ends in 'S' is what's important. If we were talking about Political Action Committees and wanted to use its acronym in plural form, we would say PACs. I don't think there would be much debate. So, to support my answer, I'm thinking of the family name 'Jones'. Now take the sentence, "The Joneses attending the party traveled from different parts of the country." The plural, not possessive, of Jones is Joneses. I'm not completely sure but I think there's some gramatical rule about a noun in its singular form that ends in 's' is made plural by adding 'es' to the end. So, I vote for OSes.

I think the best choice would be OSes. Here's my argument:

I'm not convinced that being an acronym is of any signifcance here. I believe that the fact that it ends in 'S' is what's important. If we were talking about Political Action Committees and wanted to use its acronym in plural form, we would say PACs. I don't think there would be much debate. So, to support my answer, I'm thinking of the family name 'Jones'. Now take the sentence, "The Joneses attending the party traveled from different parts of the country." The plural, not possessive, of Jones is Joneses. I'm not completely sure but I think there's some grammatical rule about a noun in its singular form that ends in 's' is made plural by adding 'es' to the end. So, I vote for OSes.