OMDoc is an XML vocabulary, which basically means that OMDocs must be valid XML (i.e. have a balanced set of element tags).
The set of OMDoc valid documents is determined by the OMDoc Specification. Certain syntactical aspects can be checked by validating documents with the OMDoc document type definition.
The XML community has recently standardized a more expressive approach to specifying well-formed XML documents: RelaxNG: The OMDoc RelaxNG Schema is currently the best validation resource