O'Brien, 68, is believed to be the first Roman Catholic bishop in U.S. history to be convicted of a felony.
He was also ordered to perform 1,000 hours of community service, and his driver's license was suspended for five years. The clergyman could have gotten as much as three years and nine months behind bars.
He was found guilty in February of leaving the scene after he hit Jim Reed, 43, with his car. He claimed that he thought he had hit a dog or that a rock had struck his windshield.
Last week, O'Brien asked Judge Stephen Gerst for probation and said he could still serve Catholics in Arizona as a priest. He also apologized to Reed's family, saying: "I know there is no one to blame for this but me."
Prosecutors asked for six months in jail and four years of probation.
O'Brien led the diocese's nearly 480,000 Catholics for 21 years, but stepped down in June after his arrest in the crash.
Just weeks earlier, prosecutors announced that he had struck a deal to avoid prosecution on obstruction charges for protecting child-molesting priests. Under the deal, O'Brien agreed to no longer handle abuse claims.
Reed was drunk and jaywalking the night of June 14 when O'Brien hit him on his way home from celebrating Mass. The crash left a giant spider-web crack in O'Brien's windshield and Reed lying in the street.
O'Brien then drove the two miles back to his house and parked in his garage.
The bishop testified that he heard a loud crash but never saw anyone in the road. The defense further argued that dim lighting, headlight glare and the victim's dark clothes made him hard to see.
Had he seen the pedestrian, O'Brien testified, "I would have stopped because that's the human thing to do. I couldn't imagine not stopping."
But prosecutors argued that O'Brien knew or should have known he hit someone. He did not call police even after a diocese official told him investigators believed the car may have been involved in a deadly accident.