Cumulus cells are essential for nutrition of oocytes during maturation. In the absence of cumulus cells during maturation, oocyte developmental competence is severely compromised. In this study, we matured bovine cumulus-oocyte-complexes (COCs) for 8 h, the cumulus cells were removed and denuded oocytes were further matured for 15 h in either the medium conditioned by the initial 8 h culture, or in fresh unconditioned medium. Denuded oocytes that completed maturation in COC-conditioned medium demonstrated better developmental potential than denuded oocytes that completed maturation in standard maturation medium. An inventory was made of the metabolites secreted by COCs into the maturation medium during the first 8 h, from 8 to 23 h, and during an entire 23 h maturation protocol; the metabolomic changes in the cumulus cells during maturation were also investigated. In maturation medium, 173 biochemical components were detected compared to 369 different metabolites in cumulus cells. Significant changes in metabolomic components were evident in maturation medium and in cumulus cells during maturation, with most of the changes related to amino acid, carbohydrate, and lipid metabolism. The importance of two detected biochemicals, creatine and carnitine, for oocyte maturation was further investigated. The presence of carnitine, but not creatine during oocyte in vitro maturation improved the developmental competence of denuded oocytes.