Shark Tank investor Kevin O'Leary says he learned his most ruthless lesson in productivity three decades ago from Steve Jobs and he's still measuring entrepreneurs against it.

What Happened: "Steve would say, ';They don't know what they want till I tell them,' " O'Leary recalled of quarterly meetings built around Oregon Trail upgrades during his stint making education software for Apple in the early 1990s.

O'Leary told Steven Bartlett's Diary of a CEO podcast that Jobs boiled success down to “signal-to-noise ratio,” an idea that a leader must lock onto the day's three to five critical tasks and ignore everything else. "For Steve Jobs it was 80 signal, 20 noise," he said. Jobs' focus was so intense, O'Leary added, that "he would email me at 2:30 in the morning and expect me to get back to him."

The Canadian investor called Jobs "not a nice guy," paraphrasing one blow-up in which the Apple (NASDAQ:AAPL) co-founder snapped, "Shut up and do what I say." Yet the approach produced “wildly successful” results, O'Leary said.

See also: Kevin O’Leary Says ‘You’ll Never Be Free’ If You Remain An Employee, But There’s Actually Nothing Wrong With It: Here’s Why

He sees only one contemporary who outdoes the late Apple chief: "Elon Musk… he has no noise,” O'Leary said. “Sixty seconds of every minute, 60 minutes of every hour, the 18 hours he's awake, it's all signal. And look what he's achieved."That level of concentration, he argued, separates “geniuses of their time” from merely competent managers.

Why It Matters: The 70-year-old software mogul said he still teaches Jobs' formula to CEOs and MBA students. "It's not your vision for next year," he told Bartlett. "It's the next 18 hours you're awake."

Musk, for one, has repeatedly heralded hard work and long hours as major factors in success. At USC's Marshall School of Business commencement in 2014, Musk urged graduates to "work super hard," recounting how he and his brother launched their first company from a rented office where they slept on the couch and showered at the YMCA.

Steve Jobs was also one who demanded relentless focus at work from every employee at Apple, according to an account by Jony Ive. Ive, at an on-stage conversation back in 2014, called Jobs “the most remarkably focused person I’ve ever met.”

Photo Courtesy: Kathy Hutchins on Shutterstock.com

Read next: Trump Vs. Bezos: How ‘Big Beautiful Bill’ May Wreck $20K EV Dream