If it existed, anti-oxygen would be an oxygen atom made of anti-matter, so consisting of anti-protons and positrons, along with neutrons
"one can mix antihydrogen and antioxygen in definite proportions and then combine them chemically (the consequences of this explosion in the antiworld may be as lamentable as in the ordinary one) and get anti-H2O, i.e. a transparent liquid substance boiling at 100 °C and being a good refresher (naturally for antipeople), in short, one that resembles in everything its counterpart, water. Similarly, all kinds of antimatter reproducing all varieties of the matter surrounding us may be combined from elementary antiparticles." (http://www.springerlink.com/content/xk17271664g4r641/ click on "PDF")