Promoting the oxygen evolution reaction (OER) near room temperature is critical to improve the efficiency of many electrochemical energy storage and conversion techniques, such as water splitting and rechargeable metal-air batteries. Nowadays, researchers have developed many non-precious metal oxides as highly active OER catalysts, including many perovskite oxides (ABO₃) of first-row transition metals such as LaCoO_3-δ (LCO), SrCoO_3-δ (SCO), and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF). However, understanding the interaction between oxides catalysts and water, which determines the stability and activity of the oxide OER catalysts, is still challenging. Here we report the systematic investigation between water and various perovskite oxides with different electronic structures, using a series of in situ characterization techniques including on-line electrochemical mass spectrometry (OLEMS), environmental transmission electron microscopy (ETEM), and pH-dependent electrochemical tests. It is find that having an oxygen 2p-band closer to the Fermi level and increasing the covalency of metal-oxygen bonds could facilitate the redox reaction of lattice oxygen in perovskites during OER catalysis. In the oxides such as SCO and BSCF with activated lattice oxygen in the OER process, we observe the evolving of ¹⁸O-labeled lattice oxygen in OLEMS, the strong pH dependency of OER kinetics in electrochemical measurements, and the structural oscillation in ETEM, which all indicate a new oxygen-site OER mechanism that makes the perovskites more active and less stable. While in the oxides such as LCO with no lattice oxygen activation, all of the above phenomena are missing, implying a stable surface with traditional metal-site OER mechanism. Observing the perovskites in situ during OER allows us to better understand the interaction between electrolytes and oxides, providing us a deeper insight into the stability and active site of oxide catalysts for OER.