Pickens reportedly died peacefully Wednesday of natural causes at his home in Texas.

You may know T. Boone for being one of the successful oil tycoons in history. You may know him for the BILLIONS he donated to charity.

And, if you're a college football fan, you might know him because the stadium at Oklahoma State University is NAMED after him! In fact, over the years, T. Boone raved about OSU to TMZ Sports!

He told us back in the day the best part of having a stadium named after him is "I'll always have a seat!"

Pickens made his fortune in energy -- he started working in oil after graduating from OSU in 1951 (with a degree in geology) and ultimately built Mesa Petroleum, which he grew into one of the largest gas and oil companies in the U.S. He became a BILLIONAIRE in the '80s.

He later honed his talents as a corporate raider -- and made TONS of money buying out companies. In 1996, he started BP Capital Management, which was also wildly successful.

As for his philanthropic work, he reportedly donated more $1 BILLION to various causes during his lifetime.

Pickens famously donated more than $500 MIL to Oklahoma State -- and the funds were almost evenly divided between OSU's athletics and academics.

Another interesting note ... Pickens had pledged to donate 90% of his net worth to charity upon his death.

Nobody knows exactly how much Pickens was worth -- or where the funds will go -- but should be VERY interesting to see how that shakes out.

He's survived by 5 children, 11 grandchildren and many great-grandchildren. Pickens had been married 5 times during his lifetime ... but was not married when he died. He had reportedly been rekindling his relationship with wife #3 Nelda toward the end of his life.

Pickens was also famous for being a host to the most powerful people in the world at his 65,000-acre ranch in Texas ... including George W. Bush, Nancy Reagan, Dick Cheney, Ted Turner and more.

Pickens also penned his 1987 biography, "Boone," and 2008's "The First Billion is the Hardest."