Amid the rapid development of the digital economy and China’s carbon-peaking and carbon-neutrality goals, digital transformation has emerged as a critical pathway for petroleum enterprises to improve quality and efficiency. However, the mechanisms linking digital transformation to operational efficiency in the petroleum industry remain insufficiently understood. Using an unbalanced panel of 45 Chinese A-share-listed petroleum firms over the 2008–2024 period, this study examines the direct effect of digital transformation on operational efficiency, the mediating roles of factor-utilization efficiency and technological innovation capability, and the moderating effects of ownership and firm size. The results show that digital transformation is significantly and positively associated with operational efficiency. Among its dimensions, digital applications exert the strongest enabling effect, whereas the digital-achievements dimension may suppress operational efficiency in the short run. The relationship also operates through two mediation channels: technological innovation capability transmits the effect significantly for both efficiency measures, whereas factor-utilization efficiency transmits it significantly for total asset turnover only. By contrast, the moderating effects of ownership and firm size are not statistically significant in the current estimates: the interaction terms of digital transformation with state ownership and with firm size are insignificant for both efficiency measures. These findings clarify how digital transformation affects operational efficiency in petroleum enterprises and provide evidence for designing more effective digital transformation strategies in the energy sector.