I was under the impression it got its oxygen from the blood. That link seems to suggest some mix of maceration (skin breaking down due to humidity), acting as

I was under the impression it got its oxygen from the blood. That link seems to suggest some mix of maceration (skin breaking down due to humidity), acting as a keratolytic (causing excess skin growth which mechanically separates the wart) and an immune component. Which makes sense - warts are caused by HPV.