Just to clarify, technically you can run Weblogic managed server without admin server, if the service on the managed server doesn't access anything from the admin server. Although I have never seen anyone doing this in a prod environment.
e.g. You can start the admin server, then start OSB server, create a proxy call it echoPS which just echoes back your input. Then you shutdown admin server. Test echoPS from soapUI, it would run just fine. You can also simply starts OSB server without admin server, you'll see a bunch of errors, but the managed server will eventually start. If you test your echoPS, it works just fine.