Last time I checked, there were only PDFs with class diagrams available on the net (downloadable for at least 140 swiss francs a piece) , but not the UML models in a native format of a database modeling tool.
These PDFs are a pain in the neck to work with.
In the appendix of some standards documents there were xml printouts. For GML there are xsd files available but they are very complex. For geographic metadata there are some xsds defined in this PDF.
Luckily, parts of this document is available here, for instance. it says at the top of each xsd file:
This file was generated from ISO TC/211 UML class diagrams == 01-26-2005 12:40:02
Are these UML models of the OGC standards available online somewhere? Maybe someone has converted these ##### xsds to a more usable format?