OK, there are two issues, one being what statements (triples) are needed in order to assert the waiver, the other being where to put them.
If there is a “landing page” for the ontology then CC Rel by Example gives a good start at documentation for what to do. It tells you the operative statement, which is
where xhv: abbreviates http://www.w3.org/1999/xhtml/vocab# .
Ideally you would assert this predicate and object for both the ontology (via its ontology URI) and the ontology version (if the version has its own URI), repeating for as many aliases as you know about. (Ontology versions are a particular feature of OWL 2, not of RDF.) You want to cover as many bases as you can. So you could end up with many statements like this.
Similarly, you want to put these statements in as many places as you can, not just the ontology file itself but also any landing page that it might have (as shown in RDFa in the ccrel-guide).
Putting statements into an RDF serialization (e.g. RDF/XML) is straightforward, as shown, if you are editing the serialization directly. But if you are using an OWL tool such as Protege, it
could be harder. Protege gives you two methods that might be used, ontology annotations and individual property assertions. You can use the ontology annotation pane to add as xhv:version to the ontology, but not the ontology version. To add individual property assertions for the ontology version you may have to put the three or more URIs in the ontology itself, which would just be tedious clutter, but I don’t see another choice.
Sadly all this work is speculative as there are no tools at present (of which I’m aware) that would pick up on the CC0 annotation. That’s not to say you shouldn’t do it, in fact I’m glad someone is willing to be a pioneer, as it will be a chicken-and-egg situation for quite a while.
In addition to expressing the waiver in RDF I would recommend writing a copyright statement in prose in an rdfs:comment ontology annotation property. The RDF statements themselves are likely to get lost or ignored, but with the rdfs:comment you have humans on your side. For wording you could use that given in the CC Rel guide or by the CC0 ‘chooser’ tool.
All of the above also applies if you’re attaching CC-BY or some other waiver or annotation, but ontologies are going to be easier to work with if they’re unencumbered, and the whole reason you wrote the ontology was so that it would be used, right?
Exercise for the adventurous reader: How does this approach fail if the httpRange-14 resolution‘s advice isn’t observed?
Thanks to Ruth Duerr for asking.