Black phosphorene is a novel two-dimensional material which has unique properties and wide applications. Using first-principles calculations, we investigated the adsorption behavior of 12 different transition metals (TMs; Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au) on phosphorene. Our results showed that all of the adsorption systems have a large binding energy. The Fe-, Co-, and Au-phosphorene systems display magnetic states with magnetic moments of 2, 1, and 0.96 μ_B, respectively, which means that these systems are magnetic semiconductors. Adsorption of oxygen molecules on TM-phosphorene was also investigated. Interestingly, all the O₂-(TM-phosphorene) systems, except O₂-(Pd-phosphorene), can elongate the O–O bond, which is critical to their application as catalysts in the oxidation of CO. We also found that the adsorption of O₂ molecules enables the O₂-(Fe-, Ni-, Cu-, Ir-, Rh-, Ag-, and Au-phosphorene) systems to become magnetic semiconductors, and it allows O₂-(Co-phosphorene) to display half-metallic state. Our results are expected to have important implications for phosphorene-based catalysis and spintronics.