Functional Size Measurement (FSM) methods are intended to measure the size of software by quantifying the functional user requirements of the software. The capability to accurately quantify the size of software in an early stage of the development lifecycle is critical to software project managers for evaluating risks, developing project estimates and having early project indicators. In this paper, we present OO-Method Function Points (OOmFP), which is a new FSM method for object-oriented systems that is based on measuring conceptual schemas. OOmFP is presented following the steps of a process model for software measurement. Using this process model, we present the design of the measurement method, its application in a case study, and the analysis of different evaluation types that can be carried out to validate the method and to verify its application and results.