Translation:There is an operating system on my computer.
Splitting hairs: are there any solid rules to the semantics of prepositions when using them for possession? It marked "My computer has an operating system" wrong: I get why, but what would the difference actually be if I replaced ar with ag here? Is it that the former is just correct, and the latter is just incorrect? (The two English possibilities feel equivalent to me.)
The two sentences are semantically equivalent to you in English, even though they use different words.
They'd be semantically equivalent to you in Irish too, even though they use different words.
Not everyone would agree with you that the two constructions are semantically equivalent in all cases, in either language, though - for example, I would say "there's an OS on that CD", but I wouldn't say "that CD has an operating system".