In this article, we report the influence of oxygen concentration in the transition-metal solvent-catalyst on the crystallization processes, morphology, and defect-and-impurity content of diamond crystals. In a series of experiments, the concentration of oxygen (CO) in the growth system was varied by adding Fe2O3 to the charge, and the other parameters and conditions of the growth were constant: Ni7Fe3 solvent-catalyst, P = 6.0 GPa, T = 1400 °C, and duration of 40 h. It is found that on increasing CO in the growth system from 0 to 10 wt %, the crystallization of diamond proceeds through the following stages: single crystal → block crystal → spontaneous crystals → aggregate of block crystals and twin crystals. At CO ≥ 5 wt %, diamond crystallizes jointly with wustite (FeO) and metastable graphite. The oxygen solubility in the iron-nickel melt is estimated at about 2 wt %. With increasing oxygen content in the system, the range of nitrogen concentrations in diamonds crystallized in one experiment significantly broadens with the maximum nitrogen concentrations being increased from 200-250 ppm in the experiment without O additives to 1100-1200 ppm in the experiment with 10 wt % O added. The established joint growth of diamond and wustite suggests possible crystallization of natural diamonds in the Fe-Ni-O-C system over a wide range of oxygen concentrations up to 10 wt %.