I have never heard anyone ever say anything about aliens only living by using the same metabolistic processes as humans... But.. there may likely be another planet out there that does use oxygen, although at a different percentage from Earths. Anyways, even if we do find "other" life forms, who's to say that they're on the same level as us? Have you ever considered the fact that ants to us are like we are to other life forms out there and maybe they don't want or can't communicate with us because they see themselves as superior?
Hope that helps!
