show that the introduction of oxygen vacancies in both ways can increase the OER activity of LNO, with the H2 annealing treatment delivering the largest effect. XPS results reveal that oxygen defects lead to the aggregation of Ni to the surface of the samples, especially in LNO-Vo' where the Ni/La ratio reaches 3.5:1. The observed oxygen vacancies accompanied with the aggregation of Ni at the surface of LNO-Vo' was evaluated to play a key role in determining OER performance. More importantly, the OER activity of LNO, LNO-Vo and LNO-Vo' decreased after 1000 CV cycles. We found that the decrease in nickel concentration on the sample surface after 1000 CV is an important factor in the decay of OER performance. Overall, our study further demonstrates the importance of investigating well-defined model systems to accurately interpret electrochemical properties in relation to electronic structure for optimized OER performance.