The entire room had their fingers crossed for Leonardo DiCaprio to finally win an Oscar on Sunday, but his biggest fan was his mom, Irmelin Indenbirken.

"I was so happy you have no idea," Irmelin told ET. "I love the fact that he had such a big audience to make the statements he did."

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During his acceptance speech, Leonardo used his time at the podium to urge people to be more environmentally conscious.

"Climate change is real," Leonardo said during Sunday's live broadcast. "Let us not take this planet for granted. I do not take tonight for granted."

After the show was over, Irmelin told us how Leonardo was feeling after finally getting a taste of Oscar gold.

"He's very happy," Irmelin said, beaming. "He's thrilled."

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This was his fifth acting nomination at the Oscars, but the long overdue win might have been made even sweeter because of the wait. ET was with Leonardo on the red carpet of his first Oscars in 1994, when he was nominated for his supporting role in What's Eating Gilbert Grape at just 19.

"Being nominated against these people is like being with the top heavyweights," Leonardo said at the time. The award ultimately went to Tommy Lee Jones that year for The Fugitive.

While his mom was his biggest advocate on the night, a close second was his Titanic co-star Kate Winslet. When they greeted each other on the red carpet, you could still feel their chemistry almost 20 years after starring in the movie that made them both household names.

"I just asked him if he was OK," Winslet told ET. "Then actually I noticed that he had one little eyebrow off, so I just put it back into place for him."

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Leonardo showed off his witty sense of humor when he went to pick up his statue at the Oscars' engraving station.

"Do you do this every year?" he playfully asked the engraver. "I wouldn't know!"