Moving to Monaco, Onassis fought Prince Rainier III for economic control of the country through his ownership of SBM and its Monte Carlo Casino.

In the 1960s, Onassis attempted to establish a large investment contract—Project Omega—with the Greek military junta and he sold Olympic Airways, which he had founded in 1957.

The Onassis family's substantial property holdings were lost, causing them to become refugees fleeing to Greece after the Great fire of Smyrna in 1922.

[9] During this period, Onassis lost three uncles, an aunt and her husband, Chrysostomos Konialidis and their daughter, who were burned to death in a church in Akhisar where 500 Christians were seeking shelter from the Great Fire of Smyrna.

He later became an entrepreneur, creating an Argentine import-export company, going into business for himself and making a fortune importing English-Turkish tobacco to Argentina.

[11] He reportedly told Maria Callas that he made his fortune in Buenos Aires by forming a shipping company used for trafficking heroin.

[11] His legacy in Buenos Aires was the creation of a shipping empire and a Hellenic Culture Fund providing youth scholarships and an academic international exchange program between Argentina, Greece, Monaco and the United States; the programs are funded and administered by the Onassis Foundation and were eventually under the managing direction of his daughter Christina Onassis.

As was then common practice in international shipping, Onassis's fleet had mostly Panamanian and Liberian flags and sailed tax-free while operating at low cost.

Onassis made large profits when the Big Oil companies like Mobil, Socony, and Texaco signed long-term contracts known as time charters at fixed prices before the stock market fell.

Led by Patrick McTaggart-Cowan, executive director of the Science Council of Canada, the Commission found that the Arrow had been operating with almost none of its navigation equipment serviceable:[15] "radar had ceased to function an hour before the ship struck; the echo sounder had not been in working condition for two months; and the gyrocompass... had a permanent error of three degrees west.

Onassis moved his headquarters into the Old Sporting Club on Monaco's Avenue d'Ostende shortly after taking control of the SBM.

[19] Monaco had become less attractive as a tax haven in the wake of France's actions, and Rainier urged Onassis to invest in the construction of hotels.

[30] In 1956, Greek airlines in general faced economic difficulties, whereby companies like TAE were affected by strikes and cash shortage.

The agreement lasted until 10 December 1974, when a number of factors (namely, a series of strikes, shortage of passengers, fuel price increase, and a law from the new Greek government forbidding Olympic Airways to fire employees) led Onassis to terminate his contract.

Onassis's time at the head of Olympic Airways is known as a golden era, due to investments he made in training and the acquisition of cutting-edge technology.

During 1974, the last year of Onassis's involvement with the company, Olympic Airways transported 2.5 million passengers and had a work force of 7,356 persons.

At the time, his ownership of Olympic Airways distinguished Onassis as one of only two men in the world to own a private airline, the other being Howard Hughes of TWA.

Stocks accounted for one-third of his capital, held in oil companies in the United States, the Middle East, and Venezuela.

He owned gold-processing plants in Argentina and Uruguay and a large share in an airline in Latin America and $4 million worth of investments in Brazil.

[32] In October 1968, amidst the Greek military junta and shortly after his marriage to Jacqueline Kennedy, Onassis announced the launch of Project Omega, a $400 million investment program that aimed to build considerable industrial infrastructure in Greece including an oil refinery and aluminum smelter.

Another Greek Colonel, Nikolaos Makarezos, preferred a deal offered by Onassis's rival, Stavros Niarchos, and the project was eventually split between them.

The failure was due partly to opposition from influential people within the military junta, such as Ioannis-Orlandos Rodinos, Deputy Minister of Economic Coordination, who opposed Onassis's offers in preference to Niarchos.

The house was, then, acquired by Onassis's brother-in-law and business rival Stavros Niarchos, who bought it for his wife, Eugenia Livanos, Athina's sister.

[40] During their marriage, the Onassises inhabited six residences: her 15-room apartment at 1040 Fifth Avenue in New York City, her horse farm in New Jersey, his apartment in Paris (88 Avenue Foch), his house in Athens, his house on Skorpios and his yacht Christina O. Onassis died at age 69 on 15 March 1975 at the American Hospital of Paris in Neuilly-sur-Seine, France, of respiratory failure, a complication of the myasthenia gravis from which he had suffered the last years of his life.

[43] The Boeing 727, which transported Onassis's remains, was later purchased for US $100,000 by an American electrical engineer and turned into an unconventional private residence in Hillsboro, Oregon.