In Sophocles' Oedipus at Colonus, is Oedipus a victim or a tragic hero?
In some sense, Oedipus might be seen as a victim; although he does not allow himself to be victimized. His two sons, Eteocles and Polyneices, are trying to get Oedipus to choose which one of them he will support in the war over the kingship of Thebes. The sons try various tactics to gain that support, but neither one is successful as Oedipus rejects them both.
As for the issue of whether Oedipus is a tragic hero, there is no doubt about the truth of this statement. One thing we need to keep in mind about the term "hero" is that this has a different meaning with respect to literature than it does in modern American society. In modern society, a hero is someone we want to be like, someone we admire. In literature, though, the hero is the focal point of the work. Obviously, no sane person would want to be like Oedipus.
In the century after Oedipus at Colonus was first produced, the Greek philosopher Aristotle, in the Poetics, offered a definition that today is frequently cited as being what we call the "tragic hero." For Aristotle, the "tragic hero" was someone who was noble, famous, who experienced a change from good fortune to bad fortune, and whose story would arouse pity and fear in the audience. One of the mythical figures whom Aristotle often mentions as fitting this definition is Oedipus.
So, Oedipus certainly fits the definition for tragic hero. I'm not so sure he is a victim of anything other than a cruel fate.