The standard pronunciation is /ɔ/; that’s the pronunciation in the TLFi; it is then the same as that of “restaure” (/ɔ:/) and “restaurateur”. Nevertheless, according to the same source, [-sto-] is a possibility for “restaurer”.There must be cases of “au” being pronounced /o/ in “restaurant”, as I find that my own pronunciation tends towards /o/.

By listening carefully to the “au” in the second source (wordreference) it can be noticed that while the sound is written /ɔ/, the actual pronunciation is very near that of /o/.

One cannot say that the AU digraph (or the EAU trigraph ) is always pronounced /o/ in French. /o/ is only the most common pronunciation.
AU/EAU is indeed almost always pronounced /o/ when standing alone (au, eau) or at the end of a word (bureau, beau). But there can be regional differences. One of the source I give states that /ɔ/ can be heard at the end of a word in some eastern parts of France, Belgium and Switzerland.

When in the middle of a word we hear more varied pronunciations according to geographical areas. For example in France gauche would be pronounced [go:ʃ] by someone with a northern accent and [gɔ:ʃ] by someone with a southern accent. But when the AU digraph is followed by the consonant /ʁ/, the usual pronunciation, notwithstanding the origin of the speaker, is /ɔʁ/, therefore [ʁɛstɔʁɑ̃] being given in most dictionaries.

The most common pronunciation, of the name Paul is [pɔl] in all parts of France, whereas its feminine counterpart Paule is pronounced [poːl]. This has a historical reason: the name used to be written Pol and the pronunciation has survived the modified spelling.

Sources:
http://andre.thibault.pagesperso-orange.fr/PhonologieSemaine6.pdf
http://research.jyu.fi/phonfr/Manuel_2011.html#6.15. (6.15. & 6.16)