"I haven't had enough of my s***," she retorted in the caption.
It wasn't the only look she wore on the night - she donned two other looks on Sunday after the red carpet.
Eilish first changed into a black asymmetrical shirt and flared pants, also by Gucci, to perform her Oscar-winning James Bond theme alongside her brother Finneas O'Connell during the awards ceremony.
And for the glamorous Vanity Fair Oscars after party, she wore a voluminous navy Simone Rocha mini-dress paired with over-the-knee combat boots.
The brother and sister duo took home the Oscar for Best Original Song for their James Bond theme No Time to Die, from the latest Bond film of the same name.
The pair thanked their collaborators and their parents for being their "biggest inspirations".
Eilish became the youngest artist to win three awards for the same song, having previously taken home a Grammy and a Golden Globe award.
The pair also became the first sibling duo to win in the Best Original Song category since Robert B. Sherman and Richard M. Sherman, who won the award in 1964 for "Chim Chim Cher-ee" from Mary Poppins.