I think it's stylistic. The anime series (To aru majutsu no index) uses the same style of "to aru <noun>" instead of "<noun> to aru". It reminds me of Latin, wh

I think it's stylistic. The anime series (To aru majutsu no index) uses the same style of "to aru <noun>" instead of "<noun> to aru". It reminds me of Latin, where you can adjust the position of phrases in the sentence for emphasis or rhythm. Personally, I think of the "to" as emphasizing the specificity of the phrase, if that makes sense. It's not just that "an OS", it's this specific OS that's being referenced by "aru".