Background <p>Poor ovarian response (POR) is characterized by low oocyte yield, poor embryo quality, high cycle cancellation rates during controlled ovarian stimulation, and poor pregnancy outcomes. This study aimed to evaluate the potential of hyperbaric oxygen therapy (HBOT) in improving ovarian response, enhancing oocyte retrieval and embryo quality, and identifying the optimal HBOT regimen for POR patients.</p> Methods <p>A pre-post cohort study included 41 women aged 20–45 with POR to compare oocyte yield and embryo quality pre-post HBOT. Restricted cubic spline (RCS) regression analyzed the relationship between HBOT and the number of oocytes. A generalized estimating equation (GEE) model assessed the impact of HBOT on oocytes, normal fertilization rate, and available embryos rate.</p> Results <p>Paired analysis revealed that HBOT significantly increased the number of oocytes, follicle output rate (FORT), follicle-to-oocyte index (FOI), two-pronuclear (2PN), available embryos, normal fertilization rate, and available embryos rate. A nonlinear relationship was found between HBOT sessions and oocyte number (<i>P</i>-nonlinear = 0.016), which showed the oocytes rose significantly after two HBOT sessions, plateaued at four to seven sessions, and showed no further increase beyond eight sessions.</p> Conclusions <p>Combining the controlled ovarian stimulation process with 4–7 sessions of HBOT could improve oocyte number and embryo quality in POR patients.</p> Trial registration <p>This is a pre-post cohort study. Clinical trial number: not applicable.</p>