<p>Aiming at the low-grade complicated niobium (Nb) resources in Bayan Obo with complex mineral paragenesis, the phase reconstruction method through high-temperature melting–cooling technology is a promising strategy for changing the occurrence mode of niobium. However, the effects of oxygen on phase reconstruction are still unclear. In this study, the effects of oxygen on the Nb-bearing phase reconstruction were investigated by turning the oxygen concentration from 0% to 100%. Through adjusting the atmosphere from argon (Ar) into oxygen (O₂), the enriched phase of Nb was transformed into calciobetafite (Ca₂(Ti,Nb)₂O₇) from loparite ((Ce,Na,Ca)(Nb,Ti)O₃). In addition, oxygen is beneficial in reducing the content of impurity elements in the pyrochlore-type Ca₂(Ti,Nb)₂O₇ phase and improving the co-enrichment of Nb and cerium (Ce). The mechanism of phase transformation under different atmospheres has been discussed through the process of crystallization. In addition, the crystallization of Fe₂O₃ under a high oxygen concentration provides a clean recovery of iron from tailings. This study found that adjusting the oxygen concentration of the crystallization process is a controllable method of niobium-bearing phase reconstruction. In addition, this study provides useful information for the phase reconstruction of low-grade complicated niobium and rare earth resources.</p>