MOST will improve software engineering by leveraging ontology and reasoning technology. To reach this goal, a seamless integration of ontology technology into model-driven software development (MDSD) will be realized, resulting in ontology-driven software development (ODSD). This concerns the integration of all involved artefacts (ontology and modelling languages, models, tools), as well as the development processes (process and reasoning guidance, traceability of models).The MOST artefact integration technology will allow for managing software development and ontology-based validation and knowledge management in an integrated platform. First, integration will address modelling languages. To be able to query and transform integrated models, a unified view of metamodels must be provided. Second, ontologies and models will be integrated so that ontologies can be used in models, and vice versa. For the first time, MOST will enable integrated model chains, transformations between models either consisting of ontologies or system models. Third, ontology-based queries will be integrated on the level of MDA transformations. Querying will support the developer to fulfil requirements and address modelling decisions. The MOST technology will considerably generalize existing infrastructures, such as the ODM, to address the issues at hand.The MOST ontology-driven, model-aware software process will be guided by process guidance ontologies that formalize the rules, conditions and actions a software engineer has to conduct in specific situations. These ontologies will formalize knowledge about software processes and help their guided execution. To be able to guide software engineers through the development of integrated model chains, traceability information will also be provided for the entire chains. With ODSD, ontology-aware software can be constructed, managed, and maintained, which will be proven by three major industrial case studies.