The Oxidative Coupling of Methane (OCM) is a promising process for converting methane directly into more valuable ethane and ethylene. In this work, high-time-resolution online mass spectrometry was employed to track the OCM reaction over a commercial La2O3 catalyst, focusing on the effects of methane to oxygen ratio, Gas Hourly Space Velocity (GHSV), and the presence of H2O and CO in the feed gas on methane conversion and C2 yield. The results demonstrated that an optimized GHSV (44640 to 93000 mL·g−1·h−1) and methane to oxygen ratio (CH4/O2= 3) would achieve the highest methane conversion and C2 yield at 740 °C. Furthermore, the introduction of 1% H2O into the reaction mixture resulted in a twofold increase in C2 yield at 650 °C, while the addition of 1% CO led to a threefold increase in C2 yield at 550 °C. A model in which only the front-end catalyst is active was also developed to show excellent agreement with the experimental data. The relationship between catalytic performance and the effective catalyst position in the catalyst bed provides important insights into optimizing reactor design and operating conditions to maximize C2 yield and selectivity in the OCM reaction.