I will give you another theory why cables sound differently.(I know this does not really answer the question, if all copper cables are virtually oxygen free, but I think the main reason behind this question was to find out, if impurities make a sonic difference in copper audio cables)According to Dr. Strassner, the founder of HMS audio cables ,it is not so much the amount of oxygen which degrades the sound in copper cables, but the amount of so-called ferro magnetic particles. According to him, he is one of the very few cable companies (probably the only one) who have their wire material checked for the amount of ferro magnetic particles, since this test is very expensive. He will reject any copper raw material with more than 6 ppm of ferro magnatic particles.
The ferro magnetic particles will align and re-align according the phase of the current and use up energy. And energy loss in a auduio cable always translates into sonic loss of the sound quality.
That is one reason why sometimes audiophiles cannot agree on the sonic performance of the same cable brand. Well, now it easy to see why.
If the manufacturer does not care and does not know the amount of ferro magnetic particles, one batch of copper wire can be fine and another batch bought month later, used to manufacture the same brand of cables, can have a far too high level and thus the cables made from this inferior copper raw material will be of lower sonical quality of course.