YAJCo is a tool for the development of software languages based on an annotated language model. The model is represented by Java classes with annotations defining their mapping to concrete syntax. This approach to language definition enables the abstract syntax to be central point of the development process, instead of concrete syntax. In this paper a case study of Oberon-0 programming language development is presented. The study is based on the LTDA Tool Challenge and showcases details of abstract and concrete syntax definition using YAJCo, as well as implementation of name resolution, type checking, model transformation and code generation. The language was implemented in modular fashion to demonstrate language extension mechanisms supported by YAJCo.