Every year petroleum companies bid hundreds of million Euro for petroleum leases, auctioned out by governments. Neglecting the great flexibility in oil ventures leads to serious undervaluation of assets. Oil companies use the so called discounted cash flow approach for evaluations of E&P-projects. This approach has worked well until todays, but has a lack of sensitivity in regard to managerial flexibility to act on changing circumstances during projects. As a result the DCF approach leads to a systematic undervaluation. In economical terms this flexibility can be regarded as a premium a company can additionally evaluate in comparison to common discounted cash flow calculation. The height of premium depends on the ability of the management to act on the flexibilities, uncertainties or risk inherent in a project. The economic and mathematical solution for this problem is delivered by the real option theory. This presented work shows an adequate method for real option valuation in E&P projects by avoiding a common critical point (total project volatility is used instead of oil price volatility), by using relationships between input parameters e.g. operational expenditures vs. reservoir volume and by introducing a transparency framework for the calculation process. The case study data for a re-calculation is taken from an OMV prospect, the Maari filed.