O'Brien may not be a the best option out there, but at this point I would LOVE two years of him as OC. That gets you through Mac's rookie deal, with a guy who not only knows the Pats system, but knows the Bama system Mac succeeded in. You then can get a proper look at who Mac is before deciding on his 5th year option, a possible extension, or whether you should move on.
My first choice is to get someone from outside the organization who's showing promise under one of the top offensive HC's, and see if you can really shake things up. But if that's not available, O'Brien is the "safest" option out there, if he's interested.