SOAP services protected by WS-Security / WS-Trust standards leverage a Security Token Service (STS) to broker the federation transaction. The STS can exchange WS-Security tokens for federated security tokens (i.e. a SAML assertion) to provide cross-domain, federated access to your web services.
Note: A SOAP web service call is essentiall a HTTP call, therefore a SOAP service can still take advantage of OAuth 2.0 to protect the call. The security will be processed and if the call is authorised, the SOAP message can be processed by the web service provider.