This paper deals with the use of online simulation on Holonic Manufacturing Systems. Concepts needed for the use of online simulation in a classical hierarchical system were already defined, the observer being the central one. The behavior's differences between both classes of systems are studied to determine the best way to adapt these concepts to this new environment. In the PROSA reference architecture, staff holons were chosen to welcome the simulation models and the observer. An application on an industrial sized Holonic Manufacturing System is described to demonstrate the validity of the approach.