AO stands for "Alles Ohne". In essence the members of this website seek to find sex partners practicing sex without condoms. This has become rather popular in Germany and Austria in particular. In most cases responsible partners exchange copies of their most recent HIV test from their local health authority ("Gesundheitsamt"). Some people might object to this for ethical reasons, but it's a legal, commercial website providing a platform to those who make purity in friction their fetish.