FOR FIVE years a centimillionaire Libyan mystery man marked for political assassination by his own government has maintained another identity in Washington as a diplomat accredited to the Embassy of Oman.

Since Jan. 10, 1975, the State Department's official "Blue Book" list of foreign diplomats stationed here has carried the name of Yehia M. Omar as the political affairs counselor for Oman.

There is nothing in the routine listing to indicate that Yehia M. Omar is actually Yahya Omar, a pro-American Libyan exile whose billion dollar dealings in the Middle East in the last decare rival those of Saudi Arabia's Adnan Khashoggi, financial advisor on the royal family's vast oil riches.

But The Washington Post has confirmed that they are the same man.

Omar's whereabouts at the moment is unknown. He may or may not be in Washington. There are conflicting reports.

Omar, reportedly believed by the Sultan of Oman and others in the Middle East to have American intelligence connections, it estimated by one American diplomat to have made $400 million in sales commissions and service fees out of Oman during one five-year period in the 1970s.

Some observers have viewed Omar as a possible replacement for Col. Muammar Quaddafi if he is overthrown.

Nailed into a wooden packing crate, Omar was smulled out of Libya on a U.S. Air Force plane during the 1969 military coup which brough Qaddafi to power.

Although some sources say that Omar managed to make his peace with Qaddafi at one point during the mid-1970s, other sources who know him in Washington say he has reason to believe Omar now is on a "hit list" of political opponents of Qaddafi's targeted for assassination.

In London, two Libyan exiles -- a lawyer and a journalist -- were shot to death in separate incidents last month.

On April 17, U.S. officials disclosed that two Libyan diplomats were expelled from Washington for distributing documents calling for the assassination of Qaddafi opponents in this country. It is not known whether Omar's name appeared in the documents.

Two separate sources who are aware of Omar's comings and goings in Washington say that he arrived here recently to stay in an apartment in the Foxhall East condominiums at 4200 Massachusetts Ave. NW.

According to one source, Omar felt "this was the safest place for him at the moment."

When The Washington Post learned last week that Omanian diplomat Omar and Libyan businessman Omar are one and the same, a State Department spokesman declined to comment and referred all questions to the Embassy of Oman.

Oman's Ambassador Sadek Jawad Sulaiman confirmed that Omar had been put on the embassy staff list in 1975, but said he didn't "know what the reason was at the time."

He knows Omar, he said, but Omar doesn't work for him or the embassy. He had cabled Oman, he said, to find out "what the situation was" in 1975 and "why it has continued."

It is his understanding, the ambassador said, that Omar has a private business office of his own in Washington.

Omar is not the only foreign Arab money-man accredited to the Embassy of Oman here. Ghassan Shaker, a Saudi-born banker and sales representative close to Omani Sultan Qaboos Bin Said, is listed by the State Department as economic affairs counselor.

In the past in Oman, Shaker and Omar have been linked as business executives.

When a source revealed two weeks ago that Omar has been living here at the Foxhall East, a reporter placed a call to him through the switchboard. An operator said she would take a message and have him call back.

A "Mr. Zimmerman," who said he was from Germany and works for Omar as a secretary, returned the call. Omar was not in the country, he said.

According to "Zimmerman," the apartment Omar has been using belongs to James Critchfield, former CIA station cheif in the Middle East.

Critchfield, who was traveling and could not be reached, now works as an oil expert here and in Oman for a company called Tetra Tech International, which has offices in Roslyn.

Yahya Omar was described in a 1975 Washington Post story on Arab moneymen as "one of the Arab world's few Horatio Alger stories."

Omar was a police office in Libya in the days of King Idris before founding a local conglomerate. One of his most lucrative businesses was buying surplus equipment from the U.S. Air Force base there and reselling it.

It was U.S. Air Force contacts who helped Omar escape from Qaddafi after the coup.

In the mid-1970s, Omar hired retired U.S. Brin. Gen. Joesph J. Cappucci, former head of the Air Force's Office of Special Investigations, to train his bodyguards in anti-terrorist techniques and equip them with the most modern security devices.

Cappucci, who says the Omar loaned himm the money originally to start his won consulting business here, came to The Washington Post last week to ask that no story be written about Omar's presence on the Washington scene.

"He's a marked man . . . he knows he's a marked man," Cappucci said. "He's closed his residence in London and moved his wife to their home in Cairo. When he was getting ready to go back to Lond recently, I told him. "Be careful,' and he said 'Allah's will is Allah's will.'"