It is easy to generate XSD from an Oracle table [DBMS_XMLSCHEMA.generateSchema], but the reverse is not as straight-forward. You might want to check out Altove XmlSpy 2006 ; it has the capability to create relational DB schemas from XSD. There is also XMLFox, though cannot vouch for it not having used it myself.
As this conversion would likely be a one-time activity, as long as the tool gives you accurate results you should be fine with any?
Not sure of your business requirements but while designing your system I hope you have given thought to storing the XML data as-is in the database, as opposed to changing it to relational form.