Espinosa died in Fuenfria Hospital in Cercedilla, just north of Madrid, where he had been undergoing treatment for chronic liver disease since March. His wife, Miriam Leiva, said she was not sure whether his remains would be repatriated.
"We had hoped the disease could be stopped, but it could not be," Leiva said. "He always wanted to return to Cuba."
Espinosa was one of 75 writers and political activists locked up in 2003 during the Black Spring, a notorious crackdown on dissent that provoked international criticism and EU sanctions lasting five years.
Little known at the time of his imprisonment, Espinosa was sentenced to 20 years but released after 19 months on medical humanitarian grounds, on his 64th birthday.
By then his name was more familiar in part thanks to the work of Leiva, who helped found the Ladies in White protest group to press for their husbands' release.
Espinosa said the government had made a mistake by locking him and the others up, and vowed they would not be silenced. The Cuban government frequently accuses island dissidents of accepting money from Washington to undermine the government, but Espinosa frequently denied being a "mercenary."
"We are nonviolent people who have not committed any crime," he told reporters in November 2004 at his and Leiva's tiny Havana apartment, always overflowing with the books, papers and statistical reports they used to write about Cuba's complex and troubled economy.
There was no apparent mention of his passing in Cuba's state-controlled media, and government officials did not respond to a request for comment.
Espinosa had suffered from liver problems for years and in the early 2000s was diagnosed with cirrhosis. He was also hospitalized in August 2012.
"My health problems are from before I was imprisoned, but the conditions of the prison contributed to making them worse," the gray-haired, soft-spoken Espinosa told The Associated Press shortly before he traveled to Spain for medical treatment.
He was born Nov. 29, 1940, in the central province of Cienfuegos, and along with many of his generation was infused with revolutionary fervor following Fidel Castro's 1959 Cuban Revolution.
He graduated with a degree in economics from the University of Havana in 1961 and began a long career of mid- and high-ranking posts in the government, including as counselor to then-Prime Minister Castro in the `60s and later as head of the powerful Office of Agrarian Reform.
Espinosa also was a member of the State Committee for Economic Collaboration, specializing in a handful of Soviet bloc nations, and did a stint as Cuba's economic attache in Yugoslavia.
He took up a position at the National Bank of Cuba upon his return in the 1980s, but increasingly found himself at odds with government policy.
According to Espinosa's account, in the early 1990s, after voicing disagreement with the country's economic policies, he was denounced by a colleague, publicly sanctioned and ultimately fired.
From his later writings, it was clear that Espinosa believed the Communist government wielded excessive control over the economy and he was a strong critic of corruption and bureaucracy.
He reinvented himself as a writer about the Cuban economy, publishing articles and books in the United States, Spain and elsewhere, and doing some work for Radio Marti -- U.S.-funded broadcasts aimed at Cuba that Havana bitterly objects to as an intrusion on its sovereignty.
"He never wavered, and made his opinion about human rights known to the world," Ladies in White leader Berta Soler said Monday.
However Espinosa also vocally opposed the U.S. embargo and economic sanctions against the island, saying it gave the Cuban government an excuse for its shortcomings and the restrictions it placed on Cubans.
Espinosa's independent, critical voice touched a generation of Cuba scholars around the world, one colleague said in a prologue to his book.
"Oscar's admirable labor in his numerous, documented and brave works on the economy and social aspects in Cuba have inspired and influenced the work of many Cuban economists in the exterior," the U.S.-based economist Carmelo Mesa Lago wrote.
Espinosa's death is the third significant loss for Cuba's tiny community of outspoken dissidents in as many years, following the passing of Ladies in White co-founder Laura Pollan in 2011 and Varela Project author Oswaldo Paya the following year in a car wreck.
"The Ladies in White are in mourning," Soler said. "We have lost a brother in arms, a man who devoted his life to a democratic Cuba."
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