As a rapidly emerging topic, the search for efficient materials owing to high optoelectronic and photocatalytic activities has become critical in order to break free from the current industry's reliance on fossil fuels. Realistic conditions were taken into account by means of density functional calculations combined with thermodynamics in order to analyze the water splitting capacity for the two-dimensional Janus HfSSe monolayer, namely its reactivity with water. We have shown that in the presence of gaseous water, half of the Se or S atoms are exchanged by O atoms, leading to the discovery of two new stable phases, HfS0.5O0.5Se and HfSSe0.5O0.5 at ambient conditions. In addition to their pristine structure, these two new partially oxygenated counterparts are dynamically and thermally stable at 300 and 700 K. In addition, our results show that these oxygenated surfaces are more appropriate than the pristine structure for optoelectronic applications. Rather than the pristine structure, they have a wide direct band gap of about 2 eV. Meanwhile, the analysis of the electronic band gap as well as the band diagram shows that the structure with a Se-oxygenated surface straddles the water redox and reduction potentials which makes it the more favorable as regards water splitting applications,.