O'Leary, chairman of O'Shares ETFs, told CNBC's "Squawk Box" Tuesday he "will not touch any of these companies."
"This is a classic misery loves company ... get together, be miserable," he said of the deal.
O'Leary predicted that people won't care in a decade which car transports them, especially if they don't own it. It's a utility, he said, a "box with four wheels."
"This sector is a miserable place to be an investor," added O'Leary, known on "Shark Tank" as "Mr. Wonderful."
Fiat Chrysler did not immediately respond to a request for comment on O'Leary's remarks.
The merger, which still requires approval by both of the automakers' boards, would make the joint company the world's third largest automaker and would push it to sell an estimated 8.7 million vehicles a year.
But O'Leary remained skeptical. "Maybe you can keep a story going on a Ferrari or a BMW. But the rest is a cattle car business."
Disclosure: CNBC owns the exclusive off-network cable rights to ABC's "Shark Tank."