Simple SummaryEarly embryo development is a dynamic and orchestrated event. Oxygen plays a fundamental role in several key biological processes necessary to sustain and complete embryonic development. Variations in the atmospheric concentrations during preimplantation development can promote several modifications in metabolic function and, consequently, biological processes affecting embryonic competence. Here, we have reviewed the impact of oxygen tension on essential embryonic traits during early development and addressed strategies to mitigate harmful effects due to unbalanced oxygen tensions. Oxygen tension, therefore, is crucial for embryonic competence acquisition and pregnancy success of embryos produced in vitro.AbstractAlthough well-established and adopted by commercial laboratories, the in vitro embryo production system still requires refinements to achieve its highest efficiency. Early embryonic development is a dynamic event, demanding suitable conditions to provide a high number of embryos with quality and competence. The first step to obtaining an optimized in vitro environment is to know the embryonic metabolism and energy request throughout the different stages of development. Oxygen plays a crucial role in several key biological processes necessary to sustain and complete embryonic development. Nonetheless, there is still controversy regarding the optimal in vitro atmospheric concentrations during culture. Herein, we discuss the impact of oxygen tension on the viability of in vitro-produced embryos during early development. The importance of oxygen tension is addressed as its roles regarding essential embryonic traits, including embryo production rates, embryonic cell viability, gene expression profile, epigenetic regulation, and post-cryopreservation survival. Finally, we highlight the damage caused by in vitro unbalanced oxygen tensions and strategies to mitigate the harmful effects.