Probably the best way to reduce RBCs' and hematocrit is to donate blood. If possible, donate Double Cells & Platelets.
Unfortunately Oxygen therapy has a paradoxical effect. After as little as 30 minutes of breathing pure Oxygen, your body will perceive a RELATIVE hypoxic state when you begin to breathe regular air - and begin to produce more Erythropoeitin and then more red blood cells. In the past I helped a pro cycling team with their off season conditioning programs. I did hours and hours of research to find any way possible to legally increase the hematocrits of the atheletes - and Oxygen therapy was one of the possibilities.
Almost all androgens will increase RBC's and hematocrit. The ones with the most pronounced effects are:
Anadrol
Equipoise
Danocrine
and good old vitamin Testosterone
Good luck in the resolution of your issue.