The MDA approach aims to ease software maintenance faced with platform and business evolution. In this approach, main development artifacts, i.e. models, are defined with the meta object facility (MOF) standard. To support collaborative development in MDA, we propose a mechanism for merging concurrent changes to MOF models. Our approach has the following novel functionality. First, as MOF models can have ordered relations, our mechanism can identify the order changes in MOF models, detect the conflicts caused by concurrent order changes, and integrate those changes. Second, as MOF models must respect multiplicity constraints, our mechanism detects the concurrent modifications that result in multiplicity violations. Therefore, it ensures the consistency of the merge result. Third, we offer a framework for building conflict resolution programs dedicated to developers' particular requirements. This framework offers a flexible and automated way for resolving conflicts. This work is a part of ModelBus, an open environment for CASE tool interoperability. Its contribution is to enable models to be concurrently modified by several developers and with different tools. ModelBus implementation is available as the Eclipse open source project, model driven development integration (MDDi).