Abstract--While a number of modeling languages for constructing multiagent systems (MASs) have been suggested, none of them is widely accepted and used. A prominent reason for this is the gap that exists between agent-oriented modeling languages and the agent-based system modeling needs, including accessibility, flexibility, and expressiveness. This paper addresses the need for such a language by proposing object-process methodology (OPM)/MAS, an agent modeling language (AML) that extends OPM with an intermediate metamodel of the MAS domain. Three case studies and a comparison to contemporary AMLs demonstrate the novelty and benefits of OPM/MAS.