You just write, "Available on OS X and Linux" (or using the logos if that's your style). Simple, professional, conveys the message without drawing flak for being some angsty kid.

Yo! I'm very happy that you didn't intend it to be like that at all. In fact, I first wrote a way snarkier comment and then I remembered the "Show HN" guidelines (and basic human decency) and deleted that comment :)

I'm impressed how something as simple as reading "only available on $NOT_MY_OS" in a nicely laid out box gives me the complete opposite emotion than what you intended. My comment got a large number of upvotes, so apparently many felt the same. And still, if you look at it, there's nothing wrong with the text in and of itself. Only on this and that OS. Fine, good to know.

So apparently, writing it with that word ("only"), with the OS logos instead of the names maybe, and so happily and centrally and proudly laid out, was what changed the perceived intent. I really thought that you guys were some kind of anti-Windows crusaders. And probably, the exact same text elsewhere, or slightly different text in that spot, would completely change that.

Damn, copy is hard!

Replacing "only" with "now" (or nothing at all) would improve the tone.