The radiant bride had her fitting done at the fashion mogul's headquarters in New York City, and documented the emotional experience in a video by Vogue Weddings.
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"Seeing the dress, I was totally shocked at my complete lack of control over my emotions," said Kate. "It's a dress that reflects the love I have for my partner. It's a reflection on my life. It's a reflection on the woman I've become."
In the video, the blonde beauty is captured gliding elegantly around the studio and admiring Oscar's creation in front of a mirror as the cameras circle around her and close up into the detailing of the dress.
Kate had a clear vision of what she wanted the gown to look like, and emphasised a clean and classic line.
"I love the dress because when you see the front it feels so minimal," said Kate. "And then when you turn around there's this dramatic, glorious fall of the fabric.
"It's very delicate and subtle. Oscar did it so beautifully. There's so much poetry and elegance to it."
The ivory, satin gown was completed with a cathedral length veil and a sweeping eight-foot-long, 16-foot-wide train. Kate saved the embellishments for her jewellery, but even then chose subtle items like the slender, studded headpiece.
The Superman Returns actress said she had only ever considered Oscar de la Renta as her wedding day designer.
"Oscar is the most legendary designer, in my opinion, ever," said Kate. "I am such an enormous admirer of him. There was no one else for me."
"Don't worry — we will make you the most memorable dress," said Oscar, at the start of the fitting as the pair sit down to talk.
"A wedding gown is probably the most special dress in a woman's life," he said. "So I'm very excited to be involved in this."
Kate also took the chance to reflect on her relationship with Michael, the love of her life, and explained that the couple had chosen Montana as their wedding venue because they both felt so at home there.
"The moment I met Michael I knew that he was the person I wanted to spend the rest of my life with," she said. "We're best friends, great partners, and I just found my match in this life. Every detail in the wedding is a reflection of our union."
"In the same manner that she fell in love with that man, I hope she's going to fall in love with that dress," said Oscar.
"Are you sure?" asked the designer, testing the actress at the end of the fitting.
"I'm sure if you're sure," laughed Kate.
"This is by far the most dramatic and exquisite piece I've ever worn," she said. "I'm so, so in love with this dress. It's been a real dream, I couldn't have found anything better."