Oct 26 (Reuters) - Oshkosh Corp's board unanimously rejected billionaire investor Carl Icahn's offer to buy the truck maker, calling the bid ``inadequate,'' the company said on Friday. The board ``unanimously concluded that Mr. Icahn`s unsolicited, inadequate, highly conditional and opportunistic offer significantly undervalues Oshkosh and is not in the best interests of all Oshkosh shareholders,'' Chairman Richard Donnelly said in a statement. Icahn, the company's largest shareholder, offered to buy the remainder of Oshkosh for $32.50 per share on Oct. 11, saying the only condition on the offer was that shareholders elect a slate of directors he would nominate. Icahn cited what he called decade-long mismanagement at the company in announcing his bid. This week, the investor named his first board nominee, picking the former chief executive officer of an Oshkosh unit that Icahn would like to see spun off.