We describe an ontology for resource sharing in integrated systems. We call this ontology the "Resource Sharing Ontology." This ontology addresses one of the main challenges for system and service integration: the management of resource sharing interactions. These interactions, whether explicit or implicit, are difficult to model and manage, but they are critical for safe and efficient system designs. Our resource sharing ontology also covers performance assessments on resource sharing for online and offline control and diagnosis. We discuss ontology extensions for specific resource sharing scenarios such as the assessment of resource sharing complexity. We provide examples of using the ontology to model applications, such as an RLC circuit and for the assessment of a resource contention complexity metric for the maintenance of a prototype hybrid vehicle.