Dissolved gas analysis (DGA) has emerged as a noninvasive diagnostic method for assessing transformer health, enabling the timely detection of incipient faults through the quantitative monitoring of evolving gas concentrations in dielectric fluids. On the basis of first-principles calculations integrated with experimental investigations, in this study, we systematically examine the adsorption characteristics and gas sensing mechanisms of different molybdenum oxide structures, particularly with respect to ruthenium doping, in response to dissolved gases such as carbon monoxide (CO), acetylene (C2H2), and hydrogen (H2) present in oil-immersed transformers. The sensitivities of molybdenum trioxide (MoO3), MoO3−x, Ru-doped MoO3 (MoO3@Ru), and Ru-doped high-defect-concentration MoO3−x (H-MoO3−x@Ru) to these gases were evaluated through adsorption energy, charge distribution, and band structure changes. Results showed that oxygen vacancies in molybdenum oxide enhance CO absorption and significantly affect electrical conductivity. Although H-MoO3−x has a high carrier concentration, C₂H₂ adsorption has a minimal impact on its electronic properties, limiting its sensor application potential. Ru atoms are most stable at terminal or oxygen-deficient sites in MoO3 and MoO3−x. The H-MoO3−x@Ru structure exhibits strong CO adsorption performance, making it a promising candidate for CO gas sensing. Overall, Ru-modified molybdenum oxide materials show great potential for detecting dissolved gases in transformer oil.