Because of the design and the extreme high temperatures of the oem cats, the oil that is passing through your exhaust is burnt as soon as it hits the cat just after the exhaust manifold.
Without the oem cats to burn the oil it will exit at your exhaust tail pipe in the form of blue smoke.
However your oil consumption will remain the same whether you have oem cats, sport cats or decats / test pipes.
Do you get it now ?