It has become a familiar sight in the homicide division of the Metropolitan police department: Joe O'Brien seated at his desk, a cup of coffee in one hand, a cigarette in the other, a case jacket in front of him.

But starting next week, that sight will disappear. Capt. Joseph Michael O'Brien, 48, head of the homicide squad, will become Inspector O'Brien, night supervisor of the entire department.

O'Brien's replacement in homicide will have large shoes to fill, in the view of the 45 men who work under him.

"Joe's just the type of guy who's easy to work with," said Jack Hill, now a sergeant in the burglary squad and formerly O'Brien's partner in homicide. "He's even-tempered, never panics. He never forces things on people, he just blends in."

With the exception of 1973-75, O'Brien has been blending into the homicide squad since 1960. Born in Boston, he joined the force in 1952 after a hitch in the Marines.

"I had two big ambitions in life," O'Brien said recently. "To be a Marine and to be a policeman. I've been lucky enough to achieve both."

O'Brien has been more than lucky. In 1973, he was assigned to the Hanafi Muslim murder case involving the deaths of seven members of the family of Hamaas ABdul Khaalis. O'Brien not only worked successfully on the case, but also gained the confidence of Khaalis.

It was that confidence that made O'Brien a kay figure in the eventual release of hostages during the Hanafi Muslim takeover of three Washington buildings last March. It was O'Brien who assured Khaalis that his family was safe and wasn't being mistreated during the three days Khaalis held hostages.

"That case wasn't any worse than any other," O'Brien remembered. "All cases are bad. The ones that are the worst, though, are those involving kids. A bad case involving a kid is as bad as you can get."

Whatever the case, O'Brien is known as the Metropolitan police's best "third man." That is the detective whoenters and interrogation after two previous detectives have failed, and obtains a confession.

O'Brien has three children himself and a wife Patricia, who has learned to deal with his old hours over the years. O'Brien is the obly captain on the force who takes a cruiser home with him.

O'Brien is reputed to have a photographic memory.

"If you ask him about any open (unsolved) case there is, he'll tell you everything there is a know about it," said one detective."

O'Brien first became head of the homicide division when he was a lieutenant in 1970. He remained in that post until June 1973 when he was promoted to captain and moved to the seventh district as a watch commander. He stayed there for 14 months before moving on to internal affairs.

But in the summer of 1975, with a spate of unsolved homicides on their hands, the police hierarchy decided to move O'Brien back to homicide. The number of unsolved homicides in Washington has diminished since then.

Now O'Brien is leaving homicide again following another promotion. "They'll all get along without me," O'Brien said with a smile then asked if he will be missed.

It appears likely that O'Briens replacement will be Capt. Arif H. Mosrie, who helped supervise the recent "String" operations. Mosrie is currently in charge ofnt "String" operations. Mosrie is currently in charge of the narcotics division.