Modern cyber-physical applications, such as those adopting the Digital Twin paradigm, typically connect simulators with data-rich components and domain knowledge, both often formalized as knowledge graphs. Engineering such applications poses challenges to developers. This paper presents a language-based integration of knowledge graphs and simulators for object-oriented languages. We use Functional Mock-Up Objects (FMOs) as a programming layer to encapsulate simulators compliant with the FMI standard into OO structures and integrate FMOs into the class and type systems. We show how FMOs can be integrated into knowledge graphs by means of semantical lifting, and used to ensure structural properties of cyber-physical applications. We provide a prototype implementation of the proposed integration and discuss how it can be realized in other languages. Finally, the use of FMOs in practice is illustrated by two case studies.