From Fox Sports’ Ken Rosenthal.

Wasting no time initiating his overhaul of the Reds, new owner Robert Castellini has fired general manager Dan O’Brien (above, left).

Castellini, who was approved as the team’s owner by Major League Baseball only last Thursday, is expected to begin a search for a new GM immediately.

Brad Kullman, the Reds’ director of major-league operations, will serve as interim GM

O’Brien’s replacement will report directly to Castellini instead of chief operating officer John Allen, who has been removed from baseball operations and now will head only the business side.

The possible replacements for O’Brien include:

Former Orioles GM Jim Beattie, the favored candidate of Red Sox president Larry Lucchino ” a close friend of Castellini’s ” during the Sox’s recent GM search.

Twins assistant GM Wayne Krivsky, who was the runner-up to O’Brien when the Reds last made a change in Oct. 2003.

Braves assistant GM Frank Wren, who was the Orioles’ GM in 1999.

Kullman, a leading internal candidate along with the Reds’ director of international scouting, Johnny Almaraz.