One of the most distinguished features of object-oriented paradigm is the modeling power. It captures real-world entities with a single concept, the object, which combines both the structural aspect and the behavioral aspect of the entities. It provides primarily the concepts of encapsulation and inheritance. Modeling concurrency is very crucial to achieve computational efficiency in case multiple concurrent activities in an application need to be synchronized. The conventional object-oriented modeling lacks the modeling features for concurrency in the sense that it is difficult to separate synchronization from the specification of methods. The separation allows the reusability of a class. In this paper, we propose a modeling technique through a diagrammatical representation of Petri nets, called Object-Relationship Diagram, for object-oriented applications. We first describe structure model and behavior model which describe structural aspect and behavioral aspect of objects, respectively. For behavior modeling, we use the notions of generalization and aggregation to provide users with comprehensibility of the model. We then extend our model to express concurrency and synchronization features to manage concurrent access to objects.