The physical properties of oxygen, in particular, the blue color of the liquid phase, the red glow of its chemiluminescence, and its paramagnetism as shown by the entrapment or deflection of liquid oxygen by a magnetic field, can be investigated in a regular school setting with hand-held spectrophotometers and digital cameras. In college-level chemistry courses, the paramagnetic property often serves as a dramatic illustration of the usefulness of molecular orbital (MO) theory. However, MO treatment of oxygen molecules alone cannot explain the observed photon absorptions and emissions. In fact, it is the formation of oxygen "dimers" (O[subscript 2])[subscript 2] that accounts for the observed optical phenomena and is also responsible for the "not-quite-right" degree of deflection of the liquid stream of oxygen near a magnet. A review of experimental and theoretical studies of O[subscript 4] and (O[subscript 2])[subscript 2] suggests that the so-called one-photon-two-O[subscript 2]-molecule mechanism be considered a one-photon-one-O[subscript 2]-dimer process. (Contains 1 table, 4 figures and 2 notes.)