not sure what you mean by ESS (http://www.all-acronyms.com/ESS) but calling a web services is pretty simple from within ODI i.e. using OdiOsCommand. We trigger different SourceSystems this way to deliver csv files (i.e. to an ftp site or via Mail), also i.e. OBIEE-iBots (Agent in 11g terminology) can be triggered this way.