Promoting the use of renewable energies and reducing the emissions caused by the consumption of fossil fuels have become significant points for the global industrial community. Green hydrogen, produced from renewable sources, is a promising energy vector, but its implementation has been hurdled by its cost premium in relation to grey hydrogen. One way to increase the economic feasibility of green hydrogen is to market the byproduct O2. In our previous work, an oxygen liquefaction process was developed to be used in a water electrolysis plant that can output liquid oxygen (LO2) with lower specific energy consumption and cost than other O2 production methods like cryogenic distillation (CD) and pressure swing adsorption (PSA). In this study, life cycle analysis is conducted for a 50 MW water electrolysis plant with an integrated oxygen liquefaction unit aiming at assessing its global warming potential (GWP) in four different energy supply scenarios of the Portuguese context. The energy and material balance were determined by simulation with the Aspen Plus software and then GWP impact was assessed using SimaPro software. The results demonstrated that adding an oxygen liquefaction unit to the electrolysis plant only increased the GWP by 6.5–6.8%, compared to the isolated water electrolysis plant. Additionally, the GWP of the O2 liquefied in the unit proved to be inferior by 34–81%, depending on the energy source, when compared to the LO2 produced at air separation units. Lastly, the cost analysis showed that for the Portuguese case, the 50/50 combination of solar photovoltaic (PV) and onshore wind energy outputs similar levelized cost of H2 (LCOH) and levelized cost of LO2 (LCOLO2) as the current grid mix, being the lower costs achieved when the contribution of solar is higher, reaching LCOH equal to 3.37 €/kgH2 and LCOLO2 equal to 0.058€/kgO2 for 80PV/20Wind ratio.