Styra DAS is the control plane for OPA. You run OPA wherever you want and then connect it to Styra DAS as its management/control plane.  (For the Free/Pro versions you can only use OPA as a Kubernetes Admission Controller, but for the Enterprise you can run any OPA integration.)  For all communication, OPA reaches out to DAS over HTTPS and authenticates with a bearer token.  (We configure all this for you if you follow the installation instructions provided by the DAS).