The xmllint tool (or equivalent library call) refuses a valid XML schema.
(or it seems to be a valid schema at least, some other validators process it just fine, availible at [1], it is a part of several interlinked schemas from [2].)

It complains about wrong base type for a type with union:
$ xmllint -schema oval-definitions-schema.xsd scap-rhel5-oval.xml >/dev/null
oval-definitions-schema.xsd:798: element complexType: Schemas parser error : 
local union type: A type, derived by list or union, must have the simple ur-
type definition as base type, not '{http://oval.mitre.org/XMLSchema/oval-
definitions-5}(NULL)'.
oval-definitions-schema.xsd:810: element complexType: Schemas parser error : 
local union type: A type, derived by list or union, must have the simple ur-
type definition as base type, not '{http://oval.mitre.org/XMLSchema/oval-
definitions-5}(NULL)'.
(... and so on ...)
WXS schema oval-definitions-schema.xsd failed to compile

Relevant XML schema snippet:
<xsd:complexType name="EntityStateBaseType" abstract="true">
     <xsd:simpleContent>
          <xsd:extension base="oval-def:EntityBaseType">
              <xsd:attribute name="entity_check" type="oval:CheckEnumeration" 
use="optional" default="all"/>
              <xsd:attribute name="var_check" type="oval:CheckEnumeration" 
use="optional" default="all"/>
         </xsd:extension>
     </xsd:simpleContent>
</xsd:complexType>
(...)
<xsd:complexType name="EntityObjectBoolType">
  <xsd:simpleContent>
    <xsd:restriction base="oval-def:EntityObjectBaseType">
      <xsd:simpleType>
        <xsd:union memberTypes="xsd:boolean oval:EmptyStringType"/>
      </xsd:simpleType>
    </xsd:restriction>
  </xsd:simpleContent>
</xsd:complexType>


References / further information:

[1]: http://oval.mitre.org/language/version5.3/
[2]: http://oval.mitre.org/language/version5.3/ovaldefinition/complete/oval-definitions-schema-complete.zip
[3]: http://mail.gnome.org/archives/xml/2010-March/msg00029.html
[4]: http://mail.gnome.org/archives/xml/2007-June/msg00107.html
[5]: http://markmail.org/message/anzfqww27wr3jlye