In this paper an object-oriented approach to modelling of chemical processes is discussed. An object-oriented modelling language, Omola, is used for dynamic modelling and simulation of chemical processes with control systems. The model representation is object-oriented and the simulation problem solving is equation-oriented. The structuring of a model into submodels in a hierarchy facilitates the development of well abstracted models. The inheritance concept, which makes it possible to specialize previously defined models into new models, facilitates the reuse of models. In the paper suggestions on both process structure and model class hierarchy guidelines are given. The idea is that these guidelines can be useful in the organization of model libraries and in the development of large process models. The paper also discusses the use of combined discrete event and continuous system simulation in Omola. It is exemplified in process control system modelling.