You need to register the package of the metamodel in the package registry first:
resourceSet.getPackageRegistry().put(BlahPackage.eNS_URI, BlahPackage.eINSTANCE);
For an UML model for example it would be "UMLPackage.eNS_URI" etc. In your case, I suppose it must something like "OpmetaPackage.eNS_URI" and "OpmetaPackage.eINSTANCE".