Today, most of the OO software development methodologies analyze requirements in terms of objects and their interactions. As a result, they have the problem of forcing analysts to make design decisions on objects and their interactions at the early stage of requirements analysis. This paper gives a brief introduction of an enhanced DFD model called data flow net (DF net) that resolves the impeding mismatch between DFD based models and OO models. And, it also gives a brief introduction of an approach that uses DF net to specify use-cases in the requirements analysis stage without the need of making the above-mentioned major design decisions. In the design stage, the proposed approach transforms the DF nets specified during the requirements analysis stage systematically and precisely into OO designs. It bridges the gap between requirements and OO design. A significant part of the transformation can be automated.