Oviducts contain various nutrients that provide energy during oocyte development. This study aimed to improve the efficiency of in vitro reproduction using extracellular vesicles (EVs) produced by the oviduct epithelial cells of sika deer (Cervus nippon). Surprisingly, the uptake of deer oviduct epithelial cell extracellular vesicles (DOEC-EVs) by cumulus-oocyte complexes, which were encapsulated by dense cumulus cells (CCs), occurred only in CCs during maturation. Therefore, we hypothesized that DOEC-EVs are transported to oocytes through CCs to exert their effects. We first investigated the effects of DOEC-EVs on the expansion capacity of the cumulus-oocyte complexes, as well as cell cycle progression, proliferation, apoptosis, and lactate and pyruvate levels in CCs, and examined reactive oxygen species levels, mitochondrial function, and key gene expression. The results showed that DOEC-EVs regulated cell cycle progression, promoted proliferation, reduced apoptosis, and improved antioxidant capacity and glycolysis, and through the oocyte first polar body excretion rate, reactive oxygen levels and mitochondrial membrane potential, it was shown that CC promoted in vitro oocyte maturation, improved the antioxidant capacity and mitochondrial function of oocytes, and promoted parthenogenetic embryo development. These results suggest that DOEC-EVs improve the efficiency of oocyte development in deer in vitro by acting on CCs, laying the foundation for further research on in vitro deer reproduction.