Creation is done by executing the command "obolib-obo2owl plant_ontology.obo -o plant_ontology.owl"
the oboformat software is available from https://code.google.com/p/oboformat/

The owl file then needs to be hand edited to include the version info. The following line goes right above the </owl:Ontology> line:
<owl:versioninfo>xml:lang="en" version ##</owl:versioninfo>
where ## is the version number, at least for the version in the live folder.  This is not done for the version in trunk.