Porous boron nitride (BN), a combination of hexagonal, turbostratic and amorphous BN, has emerged as a new platform photocatalyst. Yet, this material lacks photoactivity under visible light. Theoretical studies predict that tuning the oxygen content in oxygen-doped BN (BNO) could lower the band gap. This is yet to be verified experimentally. We present herein a systematic experimental route to simultaneously tune BNO's chemical, magnetic and optoelectronic properties using a multivariate synthesis parameter space. We report deep visible range band gaps (1.50–2.90 eV) and tuning of the oxygen (2–14 at.%) and specific paramagnetic OB3 contents (7–294 a.u. g−1). Through designing a response surface via a design of experiments (DOE) process, we have identified synthesis parameters influencing BNO's chemical, magnetic and optoelectronic properties. We also present model prediction equations relating these properties to the synthesis parameter space that we have validated experimentally. This methodology can help tailor and optimise BN materials for heterogeneous photocatalysis.