Has done well with a very poor team (wycombe), a small club in a big league (leicster) and a big club in a small league (celtic). To be honest if Celtic had beaten cheating Mourinhos Porto team, he'd have won everything possible for the clubs he was at.
Mourinho has built/bought a long ball team at Chelsea with Drogba the battering ram, just a more expensive leicster with Heskey and Celtic with Hartson. O'Neill would have won the league at Chelsea too (don't say anybody could, Keegan spent a load at Newcastle and so have Madrid).
I think he'll try and get villa into top 10 for a couple of years and maybe get a good UEFA run and then look at Manu and Arsenal who I expect will be losing their manager in the next 3-5 years.