There seems to be any amount of RDF-format for the OSLC but what I'm looking for is a simple E-R-like view of the OSLC metamodel which shows the concepts and relationships which can be used to understand the organisation and possible queries.
Is there a (graphic) representation of the OSLC metamodel anywhere?
If you are after a simple graphical diagram, you can find UML models under this Lyo-Docs repo. you can find the source .emx files, as well as .png snapshots under the folder "OSLC-V2/images/".
I you are developing OSLC applications, you might want to consider the modelling tool Lyo Designer. There, you can find a graphical model of the OSLC Core and Domain concepts. The models are based on a OSLC-specific modelling language. Lyo Designer allows you define/extend your own models, from which you can generate an OSLC application, based on the Eclipse Lyo SDK.
I here assume you are aware of the java class implementations of the OSLC Core concepts in Eclipse Lyo. There is also an implementation of the domain specifications.