This paper presents a method for estimating the size, and consequently effort and duration, of object oriented software development projects. Different estimates may be made in different phases of the development process, according to the available information. We define an adaptation of traditional function points, called "Object Oriented Function Points", to enable the measurement of object oriented analysis and design specifications. Tools have been constructed to automate the counting method. The novel aspect of our method is its flexibility. An organisation can experiment with different counting policies, to find the most accurate predictors of size, effort, etc. in its environment. The method and preliminary results of its application in an industrial environment are presented and discussed.