thats quite simple. Just open the adminstration webconsole on http://127.0.0.1:8080
Select the deployments section in the left bar and deploy the oracle jdbc jar. (e.G. ojdbc6.jar) and activate is after the wizard is closed.
Now open the "Datasource" section and use the button in the right upper corner to add a new one. The wizzard will guide you through the setup.
Very nice done!