“Oprah’s Next Act,” is the cover of the new issue of Fortune. Here you’ll find the scoop — the full-blown personal drama and trauma — around OWN: The Oprah Winfrey Network, the cable TV channel that she’s three months away from launching.

Two weeks ago, in her Chicago office, Oprah told me about her deep trepidation in taking on what will be the biggest risk of her professional life. Her 50-50 deal with Discovery Communications nearly fell apart, she explained, when Discovery CEO David Zaslav demanded that she step up her commitment to OWN and leave broadcast TV entirely.

One of the most interesting moments in the two-and-a-half hour interview was when Oprah talked about Michael Jackson. The late superstar was so paralyzed by the meteoric success of Thriller that he couldn’t risk his celebrity on a next big thing. “This is why I lived in fear about this network. I kept thinking I have to repeat the 25-year phenomenon of the Oprah show,” she told me. “I don’t want to be Michael Jackson.”

All this dish and more is in the cover story of Fortune‘s Most Powerful Women issue, which will hit newsstands on Monday. Not in the story: an amusing exchange Oprah and I had about “power.” I told her that my favorite definition of power is one she gave me years ago when I interviewed her for a 2002 cover story, Oprah Inc.: “the ability to impact with purpose.”

“I said that?” she replied. “That’s one of those lines I’d write down!”

Today, Oprah says she defines power as “strength over time.” How Newtonian. I guess if you’ve been doing what you’re doing for a quarter of a century, “strength over time” is a good definition. And something to aspire to.

When OWN launches on 1/1/11, we’ll see how Oprah fares in her “next chapter,” as she calls it. No question, though, Oprah — No. 6 on the new MPWomen list — has evolved as a businesswoman and more. She told me: “Now I accept that I’m a brand.”