Agents in open systems interact continuously, each possibly having a different ontology. Mapping in advance all the ontologies that an agent can encounter is not feasible, as all the possible combinations cannot be foreseen. Mapping complete ontologies at run time is a computationally expensive task. This paper proposes a framework in which mappings between terms may be hypothesised dynamically as the terms are encountered during interaction. In this way, the interaction itself defines the context in which small, relevant portions of ontologies are mapped. We use this way of scoping the ontology mapping problem in order to apply mapping heuristics in a more focused way.