Boron incorporation from the gas phase was achieved in MPCVD grown (100)-oriented homoepitaxial diamond layers, either with or without a small fraction of oxygen in the gas phase, in addition to hydrogen, methane and diborane. From secondary Ion Mass Spectroscopy (SIMS), it is shown that the 0.25% of oxygen decreases the Boron concentration [B] by two orders of magnitude. In this way, we demonstrate that it becomes possible to control [B] with low levels of compensation and passivation down to the 10(15) cm(-3) range. Cathodoluminescence spectroscopy is systematically performed in seventeen samples under a 10 kV acceleration voltage at 5 K and the exciton bound to boron (BE(TO)) intensity to the free exciton (FE(TO)) intensity ratio is evaluated (I(BETO)/I(FETO)). A linear relationship between I(BETO)/I(FETO) and FBI with a coefficient of 3.5 x 10(16) cm(-3) is demonstrated for [B]< 3 x 10(17) cm(-3) in single crystalline diamond, irrespective of the gas phase composition during growth.