The file containing the ontology (purl.org/ontology/wo) doesn't seems to be available as mentioned in the comments. (EDIT: It's possible to load the file via Protege: "Open as URI" --> http://purl.org/ontology/wo/
) However, the terms are listed further down the HTML page, so you could recreate it yourself if needed.
The ontology should be representable using RDFS axioms (ranges, domains, subproperties and subClasses) or using OWL. You can serialise the ontology in RDF/XML or any OWL syntax, both format are handled by the OWL-API.