Labor Secretary William E. Brock has made good on an ultimatum he delivered to the Senate Labor and Human Resources Committee last month, notifying a career safety expert that he is being transferred from the Mine Safety and Health Administration (MSHA).

Two hours before the panel voted last month on the nomination of a senior House Republican staff member to become an assistant secretary of labor for mine safety, Brock said he would remove a highly regarded safety expert from the mine agency if the committee rejected the nomination.

It did, by a 9-to-7 vote along party lines, with the Democratic majority upset over what one Senate aide described as an "out-and-out threat" from Brock.

Last week it was the secretary's turn, and he began actions to remove the career safety expert, Alan C. McMillan, from his position as acting head of the mine safety administration.

Brock named former Illinois lieutenant governor David C. O'Neal, a Republican, as deputy assistant labor secretary for mine safety and said through spokesmen that O'Neal is expected to replace McMillan as head of the agency.

McMillan, who had headed the embattled mine safety administration since January on an acting basis, is "a very talented and highly prized administrator" who will be given another important assignment in the Labor Department, said Chriss Winston, a Brock spokeswoman.

Winston said she was uncertain whether the White House would seek a presidential appointment for O'Neal as an assistant labor secretary, the level typically carried by the MSHA administrator.

If the administration decides to keep O'Neal at the current level, it can avoid another confrontation with the Senate panel because the appointments of deputy assistant secretaries do not require Senate confirmation.

A career federal worker, McMillan was the Atlanta regional administrator for the Occupational Safety and Health Administration when he was named to succeed David Zegeer, who retired.

The Reagan administration had wanted Dorothy Strunk, a member of the House Education and Labor Committee staff for 20 years, to replace Zegeer. Brock personally lobbied for her confirmation.

But the United Mine Workers and other unions opposed her selection, saying that she lacked technical knowledge and the administrative skills to manage the 2,900-employe agency, which both the Senate panel and the UMW have accused of taking a lax attitude toward mine safety.

O'Neal went on the Labor Department payroll Sunday, leaving the Interior Department's Bureau of Land Management.