I can't seem to find my Machinery's handbook -it's been quite some time since I actually did any machining- but, IF I recall correctly...
"Oxygen free" copper is copper that has been processed in such a way as to eliminate microscopic voids....little pockets of air that greatly increase the electrical resistance of the copper.
I do know that "oxygen free" copper is one heck of a lot harder to machine than you would expect!
