Maj. O.W. Sweat, the third highest ranking officer in the Montgomery County police department and the man credited with overseeing most activities of the current force, retires today.

Sweat's retirement is a major loss, police officials said, and creates a key opening in the department that is responsible for much of the day-to-day operations. A successor will not be named for at least a month.

Lt. Col. Donald E. Brooks, the second highest ranking officer after Police Chief Bernard D. Crooke, described Sweat as a "real force in making the department what it is today."

Sweat, 50, who was in charge of about 70 percent of the police force and was known for his tough disciplinary attitudes, surprised county officials a month ago when he announced that he was leaving to take over as chief of protection and security at the National Institutes of Health in Bethesda.

"I'm going out on top, with good feelings," Sweat said in an interview this week.

Brooks said that the job of chief of field services -- the position that Sweat has held for the last two years -- is "crucial" in executing policies developed by department officials. Field services include the patrol force and the traffic squads.

"They are the first responders [to reports of trouble]; they are the uniformed presence, the image-makers," Brooks said. "We don't advocate authoritarians, but there are times when authoritarian leadership is necessary. He [Sweat] has been an authoritarian in carrying out directions and ensuring that the goals of the department are met."

One area where Sweat was particularly effective, Brooks said, was in the department's campaign to crack down on drunk drivers. Police records indicate that under Sweat's command the number of drunk driving arrests for 1985 show a 30.6 percent increase over 1984.

Sweat also has been aggressive, Brooks said, in arranging for traffic officers to receive additional training. The 30 officers are now certified as vehicular homicide investigators as a result of that training, police officials said.

In addition, Sweat enforced the department's dress code, which specifies that men keep their hair short and mustaches trimmed. Women must keep their hair either pinned up or cut short.

"I believe that citizens deserve good police work and an officer who is proud of his profession and proud of his appearance; it's just part of a semimilitary organization, which is what a police department is," Sweat said.

Before joining the Montgomery County Police Department on Aug. 3, 1959, Sweat served in the U.S. Marine Corps. He liked the military life, but not the traveling assignments, which kept him away from home for months at a time.

Sweat said that he decided to enter police work because it offered a more stable life style than the military, it allowed him to "do some good for other people," and it presented a challenge.

"Just coming to work, and not having problems there, would drive me nuts," he said.

Sweat spent about half of his police career in uniform and about half as a plainclothes officer. In 1968 he was named Montgomery County's "Policeman of the Year."

During his career, Sweat served as officer in charge of the homicide/sex squad, director of the police academy and commander of the Silver Spring District. In 1980 he was promoted to major and appointed chief of the investigative services bureau. In December 1983, he was made chief of the field services bureau, where he had command of about 625 people, or about 70 percent of the department.

Sweat's successor, who must be a major, could be named early in January, according to Brooks. First, however, the department will have to promote one of its eight captains to fill the empty slot for a major created by Sweat's departure.

When a new major has been selected, Chief Crooke will name one of the department's three majors chief of field services, a job that pays $37,338 to $57,749, depending upon the officer's years of experience.