Unlikely... Steve wouldn't give them OS X, unless they buy mac. Plus, it didn't look like to have more CLI interfaces than GUI.
But our hero (Jeffy G'bloom) uploaded a Viruz on it using OS 8/9 loaded laptop, so I'm guessing, Aliens were using Pirated version of DOS (that or Windows ME without GUI), with fancy 8bit graphics.
Of-course... had they been using FOSS, Jeffy couldn't possibly infect it and earth as we know it, would have doomed!! :bleh: