In Petroleum Development Oman (PDO), the exploration opportunity portfolio is managed by a dedicated team responsible for its integrity and analysis. The team uses a customized suite of tools for volumetric and economic assessment that has been developed since 1997. In addition to their application in quality control, regular performance monitoring and reporting, the tools are used to determine annual finding targets and formulate exploration strategies. The portfolio management process in PDO has made a major contribution to the quality of prospects that are selected for drilling. Thanks to new evaluation processes and a robust suite of tools, the company has a better understanding of the composition and quality of the prospect portfolio, and is explicitly aware of the challenges and issues facing exploration. Portfolio management tools enable objective target setting and communication, thereby achieving shared ownership. The clarity brought by portfolio analyses provides a powerful focus to the asset teams' efforts to improve prospect quality. PDO is also able to quantify the consequences of changing activity levels and balance in the program, and of the limitations imposed by the portfolio. These provide the basis for ongoing dialogue with shareholders and justify the strategy shift towards frontier exploration enacted since 1998. Exploration in PDO has shifted from being opportunity-driven to being led by a portfolio-based strategy. A similar approach may be suited to other exploration ventures with sizeable portfolios.