Directed acyclic graphs are commonly used to represent on-tologies in the biomedical domain. They provide an intuitive means to formalize relations that hold between ontological categories. However, their semantics is usually not explicit. We provide a semantics for a part of the OBO Flatfile Format by extending OWL with a method to express relational patterns. These patterns are OWL axioms with variables for classes. The variables can only be filled with named classes. Addition-ally, we provide a semantics for open patterns in OWL. Our method is applicable to the OBO Flatfile Format, and provides a means to design OWL ontologies using complex ontology design patterns. Therefore, it leads not only to an integration of the OBO Flatfile Format and OWL, but extends OWL with an intuitive interface for designing ontologies us-ing complex definition patterns. A prototypic implementation and test results are available at http://bioonto.de/obo2owl.