Flue gas recirculation and steam injection are employed in hydrogen-rich gas turbines to stabilize fuel reactivity and improve cycle efficiency, introducing high water vapor content into the combustion chamber and thereby necessitating an investigation of water-fuel interactions at elevated temperatures. This study employed UV laser absorption diagnostics behind reflected shock waves to conduct in situ measurements of the ȮH concentration time-histories during the oxidation of CH₄/H₂/H₂O/O₂/Ar mixtures at pressures of approximately 1.3, 5.0, and 15.2 atm and temperatures ranging from 1225 to 1888 K with varying hydrogen blending and water addition ratios. The absorption lineshapes of the ȮH R₁(5) transition in the A-X(0,0) vibronic band were characterized after broadening and shifting in Ar at different pressures, with diagnostic center wavelengths set at 306.6868 nm (1.3 atm), 306.6874 nm (5.0 atm), and 306.6886 nm (15.2 atm), respectively. The obtained ȮH concentration time-history data were compared in detail with predictions from eight representative reaction kinetic models, and the predictive capability of the models for ȮH behavior was quantitatively assessed using the error function method. NUIGMech1.1 exhibited superior performance in predicting ȮH behavior and was subsequently selected for kinetic analysis to elucidate stage-specific micro-mechanisms and identify key reactions driving the concentration evolution. The activating effect of H₂ on ȮH behavior during CH₄ oxidation was investigated. Results indicate that higher hydrogen levels intensify hydrogen-related reaction pathways, expanding the radical pools (H, ȮH, and Ö), thereby promoting fuel consumption through Ḣ-atom abstraction reactions. Additionally, by introducing weak collision H₂O* and inert H₂O**, the thermodynamic and kinetic effects of water were distinguished. The results show that under the current conditions, H₂O primarily affects ȮH behavior through direct participation in reactions rather than through three-body collisions or thermal effects. Further selectively activating the water-containing pathways revealed that the reactions CH₄ + ȮH = ĊH₃ + H₂O, Ö + H₂O = 2ȮH and H₂ + ȮH = Ḣ + H₂O are key channels for water participation in hydrogen-rich methane combustion chemistry.