Icahn, who spoke with CNBC's "Fast Money: Halftime Report," said that Saudi Arabia's decision not to cut oil production blindsided the global market, and that he expects the commodity's price will keep sliding.
"I think [oil's price] will continue to go down unless there is some outside event," Icahn said.
Still, he said he sees a "tremendous opportunity" for when oil eventually rises again—but he admitted that nobody knows when that will happen.
"I wouldn't rush in now on oil, and that's talking against myself because I own a lot of oil stocks," he added.
Icahn did allow, however, that "there will be increased demand for oil over the years," and easily obtainable resources are dwindling, so companies engaged in offshore production will continue to gain importance. Icahn is one of the largest shareholders in drilling firm Transocean.
Icahn also revealed that he had shorted the euro/dollar trade. As the activist investor made comments on this bet, the euro's exchange rate fell below $1.14—a new 11-year low.
Icahn said he placed the currency bet about three years ago and "shorted quite a bit" around $1.27. He said he was forced to "grit my teeth" when the euro strengthened, but the trade eventually turned around.
The Icahn Enterprises chairman also discussed his activism with several companies.
EBay has "acted responsibly," Icahn said, and its board has engaged in "damn good corporate governance."
On the other hand, Icahn criticized the board of Gannett. Earlier Thursday, he owner's board and pushed for changes in the company's corporate governance practices.
"I think we might have a little fight," Icahn said. "We've been in them before, you know, and we don't shy away from them."
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