Excerpt from a notable article (from people who actually manufacture high quality cables and wiring harnesses) on oxygen free copper loudspeaker wire:
Commonly used in consumer electronics, oxygen free copper is most frequently found in high end audio-visual systems, though individual oxygen free copper wiring custom cable assemblies are also on the market for the pickiest of music connoisseurs. Oxygen free copper is used for speaker wires, audio/video connector cable assemblies, and amplifier wires because of its purported higher conductivity, and supposedly better ability to transfer low-frequency sounds. Some manufacturers also state that oxygen free copper wiring provides better clarity of sound, and lasts longer than other, less pure, copper wires.
While all of the claims about the good things about oxygen free copper wiring have a degree of truth in them, for most consumers, oxygen free copper wiring is not going to be noticeably different than any other type of copper wiring. The increase in conductivity is minimal, and is not due to the de-oxygenation of the wire. Rather, the process of electrolytic refining also removes other impurities, notably, iron, that can cause resistance. However, even so, the increase in conductivity would not be noticeable unless the wire was extended to a great length, say, over 50 feet. Since no consumer product uses wiring of anything near that length, the increased conductivity is delectably useful at best. In fact, there is no substantial difference between the performance of oxygen free copper and other types of copper in virtually all kinds of consumer products.
However, outside of the realm of cutting edge scientific equipment, and extremely advanced copper cable assemblies, oxygen free copper has little practical use, particularly in the consumer world. Though a strong advertising phrase today, it is unlikely to stick around for longer than a few marketing campaigns.
Link to article