The electrodeelectrolyte interface chemistry is highly important for photoelectrochemical (PEC) and electrocatalytic water splitting where cations in the electrolyte are often crucial. However, the roles of cations in an electrolyte are much debated and not well-understood. This work reports that the PEC and electrocatalytic water oxidation (WO) activities in basic electrolytes with different cations follow an unexpected trend (Li+ > K+ > Na+) especially for long-term reaction. Such an abnormal order of activity is found to be the balance effect of two factors: the distinct extents of the weakening of O-H bond on electrode surface after interacting with cations in different electrolytes and the different rates of oxygen reduction reaction (ORR) which turns out to be dominant. Li+ not only brings the most significant decrease of OH bond strength but also is most effective for avoiding back reaction, while Na+ shows the most detrimental effect on WO because of ORR. Our results provide important insight into the roles of cations in WO and demonstrate a new strategy of tailoring the electrode-electrolyte interface via judicious choice of cations in electrolyte for more efficient PEC and electrocatalytic water splitting.