• Official pronunciation: [o ʁə.vwaʁ] — see le wiktionnaire
• Global: the e in revoir is often omitted as in au r’voir, so revoir is closer to [rvwar]
• Belgium: a lot of people shorten the au and thus pronounce [ɔ] instead of [o]
• Orléans: au is sometimes pronounced [ɑ̃]

I’m from the south of the Poitou-Charentes (precisely often in Cognac, Saintes and La Rochelle) and as Joubarc mentions, it is also common there to hear either [ɔʁvwaʁ] or [ɑ̃ʁvwaʁ] (what I personally say).