Made by submerging a test tube in a styrofoam coffee cup half filled with liquid nitrogen then slowly passing oxygen gas through a pasteur pipette inserted into the test tube. After about a minute, the yummy looking, blue liquid had half filled the test tube, which was then held in front of the slit entrance of a 'Project Star' spectroscope, and a microscope light shown through it, producing this spectrum:

There's a lot of detail in that spectrum, including molecular orbital banding in the red and blue-green, but the major features are wide bands - gaps in the continuous spectrum - at about 620, 570, 525 and 470 nanometers (reading from the bottom scale.) It is the widest of these absorbtion bands, at 570 nm that is responsible for the blue color of liquid oxygen.
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