Environmental assessment affects negatively the decision to invest when production oil projects are evaluated under traditional analytical methods such as net present value (NPV), because the environmental requirements may generate a delay in the start-up of production. Contrarily, under a real options approach, if the delay period provoked by legal and environmental requirements is close to the investment timing (postponement period), some decisions to produce may be unaffected. Thus, the decision-making process according to the real option theory is more realistic, since it considered uncertainty over future costs and output prices, investment irreversibility, and managerial flexibility (as option to delay investment), which means that the decision is not "now or never" (as in the case of NPV), but "now or wait." This paper discussed an option model based on the stochastic behaviour of output prices, and its relationship with the optimal rules of decisions in terms of timing period. We conclude that, if this period length is longer than that of typical production delay time in an offshore oil project in Brazil due to environmental restrictions, decisions to produce are unaffected.