By STEPHEN K. DOIG

NEW YORK - A city police detective who spent six years investigating anti-Castro bombings in New York testified Tuesday that Miami businessman Eduardo Arocena admitted in September 1982 that he was "Oman" the mysterious mastermind behind the terrorist group Omega 7.

According to Detective Robert Brandt, the admission came during an eight-day period in which Arocena first gave detailed information about other Omega 7 members, Look Brandt and FBI agent Larry Wack to Miami to search for explosives, and then abruptly went into hiding for 10 months.

Arocena was captured in July 1983 in Miami, and now is on trial on 26 counts of conspiracy, murder and perjury.

Arocena, a short, pudgy, mild. looking 41-year-old, called Wack and Brandt about meeting on Sept. 24, 1982. It was just three weeks after be had spent half a day before a federal grand jury here denying any connection to Omega 7 - in the face of considerable circumstantial evidence to the contrary.

At their first meeting, in a room at the Holiday Inn Jetport in New Jersey, Brandt said Arocena would say only that he was there "representing Omar" to find out what the government wanted to know.

But the next day, after a breakfast of eggs and cheeseburgers at a nearby diner, Brandt asked Arocena directly: "Eddie, are you Omar? I want to know who I am talking to."

Arocena put his hand on his head and thought for a moment.

"Yeah. I am Omar." Brandt said Arocena finally replied.

Arocena, though insisting that he wouldn't be taped or testify in court, then signed consent forms in English and Spanish waiving his right to an attorney during the interviews.

And he began to talk, particularly about Pedro Remon, who, Arocena said, led the Omega 7 cell in the New York area.

"He told us there was a feud between himself and the Pedro Remon group," Brant testified. "He said they were going to kill him."

According to Brandt, Arocena told them details about numerous bombings and attempted bombings of Cuban diplomatic buildings and of groups and companies suspected of dealing with Castro.

And Brandt said Arocena fingered Remon - who now is in jail on contempt charges for refusal to testify before a grand jury - for at least two of Omega 7's bloodiest acts, the machine-gun slayings of Committee of 75 member Eulalio Negrin in 1979 and Cuban attaché Felix Garcia Rodriguez in 1980.

Brandt said that Arocena, who admitted ordering the murders, said that he tried to get Remon to abort the Garcia murder mission because there were no other Cuban diplomats in the same car.

"He didn't want to kill one Cuban," Brandt said of Arocena. "He wanted to kill five."

Arocena also described how he, Remon and other Omega 7 members tried to kill Raul Roa, the Cuban ambassador to the United Nations, with a remote-controlled car bomb on March 25, 1980.

According to Brandt, Arocena described how they briefly blocked Roa's parked car with a van while Remon surreptitiously placed the bomb, loaded with plastic explosive, under the car's gas tank with magnets. Roa's unsuspecting chauffeur was in the car at the time.

But when Roa came out to leave, his car bumped lightly into another one and the bomb fell off onto the street.

"Mr. Arocena had the transmitter in his hands," Brandt said. "Remon told him to blow the bomb, but some children were passing by and he decided not to blow it."

The bomb had had an earlier, also unsuccessful mission, Brandt added. It had been built in 1979 by Omega 7 to kill Fidel Castro during his visit to the United Nations, but security was too tight for the attempt.

Brandt said Arocena gave details about Omega 7 bombings at the ticket offices of the Russian airline Aeroflot, a Cuban concert at Lincoln Center, at the Mexican consulate, and at the TWA terminal at Kennedy International Airport, among others.

Over the next several days, in New Jersey, then at FBI headquarters in New York, and finally at a Ramada Inn in Miami, Brandt said Arocena continued to cooperate. In Miami, he talked with local agents about a string of Omega 7 bombings there and promised to help find a claimed 600 to 800 pounds of explosives cached by Omega 7.

But on Sept. 30, 1982, Arocena failed to show up at the Miami hotel room for another meeting with agents. The next day he called Brandt and Wack.

"He said 'I'm going to run.' " Brandt recalled. "And he hung up."

The trial will resume today with cross-examination of Brandt by defense attorneys Humberto Aguilar and Luis Fernandez.