Kevin O'Leary, chairman of O'Shares ETFs and an investor on "Shark Tank," told CNBC on Monday he dumped Apple stock within "seconds" of the tech giant's announcement last week that it would stop revealing device unit sales.

"Don't ever tell me on a call, I don't care who you are, what I should know and what I shouldn't," he said on "Fast Money Halftime Report." "I'll decide what's relevant."

Apple is a "closed environment" and it has to sell hardware to add and retain services customers," he argued.

"What do you mean you're not talking unit sales?" he said, incredulously. "If I don't know how many you're selling, I don't know anything about you anymore."

During last Thursday's post-earnings conference call, Apple CFO Luca Maestri said the company would no longer report how many iPhones, iPads, and Macs it sells each quarter, beginning with the December quarter.

"A unit of sale is less relevant for us today than it was in the past," Maestri said.

On Monday, O'Leary revealed, "It was seconds after that, I exited the stock," adding he doesn't own it anymore. "And I'll tell you why: transparency."

Since last Thursday's announcement, shares of Apple have dropped about 9.5 percent and drawn two downgrades from Wall Street analysts.

"It's going to take a lot to get me back in," O'Leary said. "This is going to go south now because everybody's going to try to figure out, 'How do I measure.'"

— Disclosure: CNBC owns the exclusive off-network cable rights to "Shark Tank."