Green hydrogen is a potential decarbonization option for oil refineries, where hydrogen is already used in hydrotreating and hydrocracking. While previous studies have examined hydrogen production economics, limited work has integrated refinery simulation, hydrogen supply optimization, and refinery margin analysis for a UAE refinery. This study evaluates the techno-economic impact of replacing grey hydrogen with green hydrogen in the UAE's refinery operations, using the Ruwais refinery as a case study. The methodology combines CAESAR refinery simulation, green hydrogen supply-system modeling, and refinery economic and emissions analysis. CAESAR estimates refinery operation, product output, utility use, and hydrogen demand, which is used to size an off-grid solar-based green hydrogen system. The refinery requires 270,2 kt/yr of hydrogen. Meeting this demand requires 9.4 GWp of PV, 5.5 GW of electrolysis, and 824.6 tonnes of hydrogen storage, resulting in an LCOH of 6.32 USD/kg H2. Switching to green hydrogen reduces the refinery margin from 12.1 to 7.1 USD/bbl and avoids 2.7 MtCO₂/year. The results suggest that refinery-scale green hydrogen can substantially reduce emissions, but remains economically challenging under current technology costs, requiring carbon pricing, policy support, or future cost reductions to become competitive.