Last week Icahn made it clear that he would walk away from his bid if the threshold wasn't met. The tender offer expired on Monday, with about 22 percent of the shares tendered.
Icahn, who is known for buying stakes in struggling companies and then shaking them up with mixed results, had previously maintained that Oshkosh Corp. needs new management and a new strategy.
But he had said that he would "move on to other endeavors" if he did not get at least 25 percent of Oshkosh's shares in the tender offer.
Oshkosh's stock fell 2.8 percent in premarket trading Tuesday.