Wenzel, who has served as deputy commissioner since 1998, is the highest-ranking career official in the IRS. He manages the day-to-day operation of the agency and has ultimate responsibility for management and oversight of the nation's tax system.
Wenzel joined the IRS in 1963 as a revenue officer in Chicago.
"We're fortunate to have such an experienced person on board," said O'Neill. "Bob will provide continuity of leadership for this transition period, and keep the IRS on the course Commissioner Rossotti charted."
The IRS Oversight Board, a seven-member panel charged with recommending candidates for the top tax job, sent two names of potential nominees to replace Rossotti to the White House earlier this year. Board Chairman Larry Levitan said in February that he hoped the White House would nominate, and the Senate would confirm, a new commissioner before Congress' August recess. But the White House has yet to announce a nominee.