Didn't know that was a thing. The stuff marketing teams come up with boggles the mind.
As far as I can tell that is just a fancy term for most high quality copper. Most copper is recovered using electroplating which will have nearly zero oxygen content. 99.99% pure is the typical goal. Oxygen free just indicates the alloy bears less than .001% of oxygen. Basically they are saying that the copper they buy from the supplier is typical...
Different copper alloys behave differently. Pure copper will behave chemically as such. I would think it would be less durable then some copper alloys, physically at least, but that does offer the best cooling potential of copper. I doubt it will change its resistance to corrosion and oxidation from air that much compared to, well I'm not sure what. Even reclaimed copper is often recovered using acids and electroplating.
I think it is safe to assume that any product made from copper will be oxygen free. Considering that most water cooling equipment relies at some point on brass/copper/solder you will never end up with a pure environment. That is why stabilizers, biocodes, and anti-corrosives are used.
For myself, I use a little biocode and a piece of silver to keep plain distilled water free of crud. (Which reminds me, I should probably flush mine in the near future, been a while.)