This paper investigates the existence of ownership effects in the global oil and gas industry, i.e. whether there are systematic performance and efficiency differentials between National Oil Companies (NOCs) and privately owned International Oil Companies (IOCs). After discussing key issues of comparing' State Oil' and 'Private Oil', I summarise important trends emerging from the dataset, which covers 1001 firm observation years over the period 1987-2006. Using panel-data regression analysis it is shown that NOCs significantly underperform the private sector in terms of output efficiency and profitability. They also produce a significantly lower annual percentage of upstream reserves, although this may not be an indication of firm efficiency. Overall, this paper suggests that a political preference for State Oil usually comes at an economic cost. (C) 2009 Elsevier Ltd. All rights reserved.