This study analyzes the institutional and structural factors which impact on the technical efficiencies of Oil Companies. In order to identify the variables that induce technical inefficiencies of oil companies, we make use of Stochastic Frontier models using data from “Top 100 Ranking the World Oil Companies" by Energy Intelligence from 2000 to 2008. Our empirical findings suggest that government ownership adversely affects on firm efficiency which is consistent with those of prior researches such as Al-Obaidan and Schully(1991), Hartely and Medlock(2007) and Wolf and Pollitt(2008). However, vertical integration and firm diversification are positively, but weekly, associated with firms efficiencies, depending the dependant variable employed. These results largely imply that oil market liberalization is likely to improve market performance.