how is O2 transported to Complex IV active site?

Basically subj.

It looks like traditional explanation of oxygen reduction assumes
that new O2 will get between Cu_b and a3 immediately after OH- is
removed.
But where will this oxygen come from? What is the oxygen transport
mechanism to the site? Is it a plain diffusion, or there is some
preferential pathway?

Regards,
Yevgen
.