Fairfax County Executive Robert J. O'Neill Jr. will resign today as head of Virginia's largest local government, abruptly ending a two-year tenure in which he often was frustrated in his efforts to revamp Fairfax's 11,000-employee bureaucracy.

O'Neill said yesterday that he plans to leave his $145,000-a-year job in January for "an offer that I couldn't pass up" to head a nonprofit government think tank called the National Academy of Public Administration, a post that will provide him with a national stage to give lectures on local government reform.

O'Neill already had a national reputation as a government manager when he came to Fairfax from the city manager's post in Hampton, Va., and was told by the 10-member Board of Supervisors to make wholesale changes in their government.

He tried to do just that, but his efforts to link employee pay to job performance, merge duplicate county functions and hire new top managers were resisted by some county employees and interest groups. Other efforts, including programs to revitalize decaying parts of the county, were more successful.

O'Neill occasionally found himself being second-guessed by some of the very people who were demanding results -- Fairfax supervisors. For the most part, his legacy in Fairfax amounts to incremental shifts in the government's organization and slow progress toward more significant changes.

Still, most of the county's political, civic and business leaders had continued to support O'Neill, and many were shocked this weekend to learn that he was resigning. O'Neill said his departure is not a reflection of frustration with county employees or the supervisors who hired him, but he acknowledged that he is leaving at a time when his work in Fairfax is unfinished.

"Any time that you are involved in a process of change, there will be some frustration," he said. "Fairfax is a very political environment, so change is going to be tough, and it's going to take a long time. I don't think that's anything that surprised me."

Board Chairman Katherine K. Hanley (D), who had championed O'Neill for the Fairfax job two years ago and was one of his biggest supporters on the board, said she was "heartbroken" that O'Neill was leaving.

"When you bring someone with a national reputation to Fairfax, you get national attention," Hanley said. "That's what happened here."

Shortly after joining Fairfax's government in August 1997, O'Neill proposed merging the county's Park Authority with the Department of Recreation, a move that parks officials objected to because it would shift much of their authority to O'Neill. The executive had to back off after it became clear that county supervisors were worried about the political fallout from such a move.

Last year, supervisors embarrassed O'Neill by forcing him to restart the search for a police chief after Republican supervisors criticized the initial search.

And this summer, several supervisors blasted O'Neill for leaving the county for a conference in North Carolina on the day that a truck filled with gunpowder overturned at the Springfield interchange. O'Neill's absence came to light when he asked the county to pay for a $2,000 set of golf clubs that were stolen from his county car during the conference.

O'Neill quickly discovered that in Fairfax, supervisors -- particularly the four-member GOP minority -- often are not willing to cede too much authority to hired managers. Two Republican supervisors -- Robert B. Dix Jr. (Hunter Mill) and Michael R. Frey (Sully) -- frequently attacked O'Neill and his deputies for policies they said were misplaced.

"I wish him well," Dix said of O'Neill yesterday.

O'Neill "came to Fairfax thinking he could be the administrative head of the government and the supervisors would utilize his services. It didn't work that way," said Fairfax developer John T. "Til" Hazel. "O'Neill was trying to reform a government. The forces against reform are always more ingrained than the forces for reform. His advice was not always taken and was frequently ignored."

O'Neill's low point was last Thanksgiving, when Republicans accused him of conducting a sloppy search for a police chief. They criticized the process of interviews O'Neill had set up, pointing out that none of the semifinalists was a minority, and questioned the qualifications of the finalists.

Under pressure from its four Republican members, the board voted in closed session to re-interview about a dozen candidates, effectively throwing out the finalists that O'Neill had recommended. In the end, the board hired J. Thomas Manger -- O'Neill's preferred candidate -- but not before embarrassing the county executive and drawing the ire of the Fairfax Chamber of Commerce, which accused supervisors of micromanaging.

Todd Stottlemyer, last year's president of the Fairfax chamber, said the police chief's hiring must have been a blow to O'Neill.

"The police chief search was a tough experience. He really believed what he did was right and in the best interests for the county," he said. "There had to have been a sense of frustration on his part that would lead him to take a look at something else."

Like Stottlemyer, many in the county praised O'Neill yesterday.

William B. Bailey, president of the Fairfax County Federation of Citizens Associations, praised O'Neill's communication skills, his efforts to consolidate county tasks and his success in helping to forge a better relationship between the county government and Fairfax's school system.

"It's a great loss to Fairfax County," Bailey said. "Not only does he have new and innovative ideas, but he really has a grasp on what needs to be done here."

County Supervisor T. Dana Kauffman (D-Lee) praised O'Neill for his revitalization efforts. He called O'Neill "the best thing to happen to older parts of the county."

Supervisor Gerald E. Connolly (D-Providence) vowed to keep pressing for O'Neill's merit-pay plan, among other ideas. Though Connolly had been critical of the pace of change under O'Neill, he said O'Neill deserved more time to implement the changes.

"I think there's still a strong majority on the board to move forward with those reorganizations," Connolly said.

Overall, O'Neill said, "I would be surprised if people describe [my] relationship [with the supervisors] as anything but a good one."

Jonathan Howes, the chairman of the Board of Trustees for the National Academy of Public Administration, said O'Neill was the natural choice to lead the group.

"He stands out in America, I think, as a person who deals in an innovative way with providing government services," Howes said. Academy officials would not say how much O'Neill will be paid as their president.

O'Neill's departure date in Fairfax has not been set; he is to start his new job Jan. 17.