On Thursday, the actress and producer, 37, was spotted out and about in Los Angeles wearing a black hoodie embossed with the words "Treat People with Kindess," which is the the title of a song and music video released by Styles earlier this year.
Wilde paired her hoodie (which is available to shop now on the singer's website for $54.99) with black leggings, black sneakers and orange-tinted sunglasses.
"She was so happy and dancing all night!" the eyewitness told the outlet. "It was very cute. She was singing along to almost every song and having a good time with the people she was with."
Rachel Murray/Getty; Dia Dipasupil/Getty Olivia Wilde; Harry Styles
In January, PEOPLE confirmed the couple had begun dating after sparks flew on the set of their upcoming thriller Don't Worry Darling. They've since grown close, attending a friend's wedding in the winter and vacationing in Italy in July.
In August, they reached another relationship milestone — wearing coordinating couples outfits — when they were spotted walking around after a lunch date in matching ripped jeans and white T-shirts. Styles chose light-wash oversized denim with holes at the knees and a vintage Bestie Boys band tee, while Wilde opted for a black pair of distressed, ripped jeans and a plain white T-shirt with rolled sleeves.
"They seem very serious and happy," the source added.
Wilde was previously engaged to actor Jason Sudeikis, with whom she shares son Otis, 7, and daughter Daisy, 5.