No, the WSDL and XSD for this are not in OER currently since it catalogs only the webservices Fusion Applications contains, but it seems useful addition so I'll see if we can get them added.
If you have an on-premises instance you can find it in Enterprise Manager, going to the crmCommon weblogic server, from the menu selecting Webservices, and finding the OutboundMessageServiceSoapHttpPort from the list under J2EE Intrastructure Webservices. Also you can use the Test Webservice feature in EM to shows and parse the WSDL at http://[myinstance]/crmCommonSalesParties/OutboundMessageService?wsdl
This is the same format for the SOAP message triggered by the Object Workflow and sent to the remote system endpoint.
Hope this helps
FA Developer Relations - blogs.oracle.com/fadevrel