Purpose Post‐ovulatory aging causes a high frequency of aneuploidy during meiosis II in mouse oocytes, irrespective of maternal age. In this study, we evaluated the effects of post‐ovulatory oocyte aging on the protection of chromosomal cohesion involved in aneuploidy and verified the relationship between PP2A or SGO2 expression and the phosphorylation level of REC8 in oocytes. Methods Murine ovulated oocytes were incubated for 6 or 12 h in vitro after collection and denoted as the aged group. The oocytes examined immediately after collection were used as the control group. Immunofluorescent staining was used to detect the localization of PP2A, SGO2, BUB1, AURORA B, and MAD2 in the chromosomal centromere. Immunoblotting was used to quantify the expression of proteins describe above and REC8 in the oocytes. Results PP2A expression involved in the de‐phosphorylation of REC8 decreased over time in oocytes, suggesting a deficiency in PP2A in centromeres. This indicated an increase in the level of phosphorylated REC8, which destabilizes centromeric cohesion in oocytes. In contrast, SGO2 expression was significantly high in aged oocytes. Conclusions The findings show that post‐ovulatory aging destabilizes the centromeric cohesin protection in oocytes and can cause aneuploidy, which is often observed in aged oocytes during meiosis II.