Electrochemical CO2 reduction to multicarbon products provides an attractive route to store intermittent renewable electricity as high value-added chemicals. Oxide-derived Cu (OD-Cu) has been widely investigated for its tunable selectivity toward multicarbon (C2+) products; however, it still remains a challenge to understand and regulate the retained oxygen of OD-Cu in the complex reconstruction process. In this work, we investigate thickness determined residual oxygen in OD-Cu, using CuO nanosheets as prototype precatalysts. When the thickness of CuO precatalyst decreased to 1.6 nm, the enhancement of the ability to retain oxygen are achieved, leading to selective C2+ production with Faradaic efficiency of around 80% over a wide current density range of 300–700 mA cm−2 with a peak value of 84.6% at 700 mA cm−2. Long-time molecular dynamics simulations reveal the enhanced stability of Cu–CuO structure with the layers of removed oxygen increased, favoring *CHO formation and *OC-CHO coupling toward C2+ products; structural characterizations and electrochemical results further demonstrate the reconstructed stacked nanosheets with high oxygen retention capacity and easily reoxidized metallic Cu sites. This work underscores the crucial role of the retained oxygen for the OD-Cu performance and provides insights into designing OD-Cu with oxygen retention to enhance C2+ products formation.