Two‐dimensional (2D) transition‐metal carbides (MXenes) are widely adopted as potential electrocatalysts for the hydrogen evolution reaction (HER) owing to their metallic conductivity, rich tunable surface chemistry, and atomic thickness with highly exposed active sites. Previously published theoretical results indicate that MXenes functionalized entirely with oxygen have lower ΔG_H* for HER. However, MXenes contain many terminal F groups on the basal plane, which is detrimental to the HER. Herein, the development of an ultrathin Ti₃C₂ MXene nanosheet fully functionalized with oxygen is reported for the HER. The obtained oxygen‐functionalized Ti₃C₂ (Ti₃C₂O_x) exhibits a much higher HER activity (190 mV at 10 mA cm⁻²) than that of Ti₃C₂T_x (T=F, OH, and O). The improved HER performance is attributed to the highly active O sites on the basal plane of Ti₃C₂T_x MXenes. This study paves way for electrocatalytic applications of MXene materials by tuning their surface functional groups.