In a report published ahead of the annual gathering of the world's financial and political elites in Davos, the anti-poverty agency announced that globally, the richest one percent of the population now owned more than the rest combined.
And Spain - where a devastating financial crisis has eased off thanks to stringent spending cuts, tax rises and controversial labour reforms - is the OECD country where inequalities have risen the most since 2007 after Cyprus.
The Organisation for Economic Cooperation and Development counts 34 member countries, including many of the world's most advanced countries but also emerging countries like Mexico, Chile and Turkey.
Oxfam said inequalities had soared in Spain "almost 10 times more than the European average - and 14 times more than in Greece."
"In 2014, 29.2 percent of the Spanish population was at risk of poverty or social exclusion, 2.3 million more people than in 2008, exceeding the EU average by more than six percentage points."
According to the aid group, the wealth of the 20 richest people in Spain rose 15 percent last year, while the assets of the 99 percent remaining population fell by the same percentage.
It said inequalities rose due to "a combination of a huge salary gap with a backward tax system that doesn't much burden those who have the most."
"There has been a collapse in the average salary in Spain, with a 22.2 percent drop between 2007 and 2014-15," the group said, adding that the presidents of the 35 main listed companies "earn 158 times more than the salary of an average worker."
Oxfam said that tax evasion was also rife in Spain.
"Investment from Spain to tax havens rose 2,000 percent in 2014," it said.
Spain is currently locked in a political impasse after inconclusive general elections in December resulted in a hung parliament in which a ruling majority cannot easily be formed.
Rising inequalities were one of the major themes of the election campaign, with the ruling, conservative Popular Party criticised for its stringent austerity measures.
Oxfam called on the next government to make the fight against poverty and tax evasion priorities.