Oxygen free copper would not make a measurable difference to the conductivity of a battery lead, , you are confusing it with snake oil regarding speaker leads, where it doesn't make any difference either!
Common copper cable is 99.99 free anyway, and has been for a long time
As the two position ignition switch serves as an isolator for the rest of the electrical system it would seem to be the first port on call when trying to track parasitic drain,
The quantities involved are probably going to need decent measuring equipment to detect, which few amateur's will have,
And if/when they do it will probably be a fraction of the amount a battery normally looses anyway,
In sixty years of mucking about with bikes I have never experienced it so I have no hands on experience.
Just the basic "mechanics" seems to work fine for me, large battery, good connections, low resistance cables, good earths and a decent bit of overcapacity everywhere seem to work well , and when something else fails the magic smoke usually leaves some sigh of its departure!