Ontology is defined as an explicit specification of a conceptualization and can be used to provide a shared understanding of a domain which is necessary to overcome differences in terminology from various sources. To build high quality ontologies, developers are required to choose and follow a suitable methodology containing a series of steps, activities and guidelines organized in a systematic way. In this paper, we develop guidelines on how to select ontology merging or integration which has not been explored in any ontology research. As it is more cost effective to build an ontology reusing existing ontologies (integration and merging) than from scratch, we therefore propose an ontology development methodology, MIOD, which incorporates and performs ontology integration in its development process. A semi-automatic key term evaluation system is developed to find suitable source ontologies from a group of candidate ontologies. The proposed methodology and system are used to address the insufficiency of current ontology development and integration methodologies.