In order for ontologies to be broadly useful to the scientific community, they need to capture knowledge and expertise of multiple experts and research groups. Consequently, the construction of such ontologies necessarily requires collaboration among individual experts or research groups. Support for such collaboration is largely lacking in existing ontology development environments. We describe some initial steps towards the development of a collaborative ontology development environment. Specifically, we describe an ontology editing tool COB Editor which exploits the notion of modular ontologies (or ontology packages) to support sharing, reuse, and collaborative editing of partial order (i.e., DAG-structured) ontologies. COB Editor can engage diverse and relatively autonomous communities of biologists in the process of creating the ontologies needed for annotating, integrating, and analyzing diverse sources of 'omics' data.