Companies such as Apple, Pfizer, Chevron, Goldman Sachs, Walmart and General Electric moved a sum larger than the combined economic output of Canada, South Korea, Australia, Russia and Turkey out of the reach of tax collectors.
The same firms spent huge sums lobbying the U.S. government for tax breaks, receiving an estimated $1,200 in tax deductions for every $1 spent petitioning for tax cuts.
The report, titled Rigged Reform, revealed that "big U.S. companies are increasing their use of tax havens and boosting their investment in political lobbying in order to push for even greater tax breaks," Oxfam said in a statement.
The 50 largest U.S. companies used a "secretive network" of 1,751 subsidiaries in tax havens to store about $1.6 trillion offshore in 2015, according to the report.
The offshore figure was $200 billion higher than in 2014, the U.K.-based charity said.
"Tax reforms proposed by President [Donald] Trump and leaders in Congress could give these 50 companies a windfall of between $312 and $327 billion on the profits they hold offshore, in addition to massive financial benefits resulting from a dramatic cut in corporate rates and other more favourable tax treatments," the report said.
It warned that Trump's proposed tax reforms would further tip the U.S. tax system in favor of the rich and powerful and "intensify the destructive global race to the bottom on corporate tax -- at the expense of poor communities in the U.S. and around the world."
During 2009-2015, the firms spent $2.5 billion on lobbying the U.S. government.
"An estimated $352 million was spent lobbying on tax issues -- helping to secure over $423 billion in tax breaks," Oxfam said.
Nearly a quarter of tax lobbying was carried out by General Electric, Verizon Communications, Comcast, AT&T and Exxon Mobil.
"These companies have deepened their use of tax havens and increased efforts to build influence to push for even greater tax breaks than they already have," Ana Arendar, Oxfam's head of inequality, said.
She added: "Corporate tax dodgers cheat the U.S. out of approximately $135 billion in unpaid tax revenues every year and poor countries out of an estimated $100 billion annually."
