That appeared somewhat evident on Friday during his introductory press conference, when Castellini offered no public endorsement for O'Brien.
"We didn't want to complicate Friday with this announcement," Castellini said early Monday evening during a press conference at Great American Ball Park. "This was a very difficult decision. It was very difficult on Dan, who gave of himself totally here. He tried very, very hard. The results haven't materialized yet and I just wanted my own person."
No further front office purges appeared imminent. When asked if the jobs of manager Jerry Narron and chief operating officer John Allen were safe, Castellini replied "absolutely" on both.
O'Brien was named Cincinnati's GM on Oct. 27, 2003, and was entering the final season of a three-year contract. The Reds struggled under his tenure, going 149-175 (.460) the past two seasons and finishing no better than fourth place in the National League Central Division.
A 73-win 2005 season with a fifth-place finish was particularly rough to take for the club, which had high hopes entering the year because of several offseason improvements. O'Brien attempted to upgrade the pitching staff by trading for Ramon Ortiz and signing free agent Eric Milton to an expensive three-year contract worth $25.5 million.
Both moves backfired when the pitchers struggled. O'Brien fired manager Dave Miley in June and replaced him with Narron, who was signed through the 2006 season.
O'Brien was informed of the decision during an 11 a.m. meeting with Castellini. When reached Monday night, O'Brien was still trying to reflect on what happened.
"Whenever you have a new owner, it's their prerogative to make changes and have their own personnel," O'Brien said. "It's a difficult part of the game but I respect the decision. With that being said, I was somewhat surprised."
Castellini planned to begin reaching out to potential candidates to replace O'Brien on Tuesday, but did not identify anyone as specific possibilities other than Kullman and Jim Beattie, a former Orioles executive vice president.
Castellini revealed that the club would be hiring Beattie as a special advisor later this week.
"What I want him to focus on while he's here is to take a look at our entire pitching program, from soup to nuts, and see what our plan is and let me know," Castellini said. "Because I haven't been able to fully understand what our plan is."
The new owner said he preferred to hire someone with experience as GM or in other top baseball operation capacities. He also wanted the new GM to "establish a culture of winning" for his team and identified clubs like Atlanta, Minnesota, Boston, St. Louis and the White Sox as other organizations with people he admired.
"It's got to be somebody who's very knowledgeable about baseball, passionate, a good evaluator of talent," said Castellini, who was a minority owner of the Cardinals before taking over the Reds.
Twins assistant GM Wayne Krivsky finished second to O'Brien the last time there was an opening and has been a past contender for other GM jobs. Twins GM Terry Ryan told MLB.com the Reds had not contacted him yet for permission to speak to Krivsky.
Castellini also admitted he was seeking a GM that would be dynamic in dealing with the public.
"It's important we can convey our message through the media and to the fans," Castellini said. "I think for a long time, the fans and the media perhaps felt that hasn't always been the case."
The timing of the change puts the Reds in a difficult spot with the start of Spring Training less than four weeks away. Castellini planned to interview six to eight candidates with the process taking "three to four weeks." The club has arbitration cases pending with first baseman Adam Dunn and shortstop Felipe Lopez next month and is still seeking to acquire another starting pitcher and a closer.
Therefore it will have to be close to business as usual for the Reds under Kullman, who said his team had been receiving trade interest from clubs for outfielder Austin Kearns. He said other deals would be explored too, although Castellini would always be consulted.
"We'll consider anything," Kullman said. "I wouldn't be surprised if in the next week we try to do something. Whether it happens or not, I don't know."
Kullman was the Reds' assistant GM from 2002-03 before becoming director of baseball operations before the 2004 season. He and special assistant Leland Maddox filled former GM Jim Bowden's duties when Bowden was fired in the middle of the 2003 season.
"We'll be bringing in all of our top evaluators and advisors at the end of the week here," Kullman said. "We're going to really sit down and have some knockdown, drag-out meetings to an extent. We need to be honest with ourselves and each other of where we are as an organization and what could be done realistically to give Bob, now inside the organization."