The 32-year-old singer and 35-year-old reality star revealed they're back together by sharing photos from bed Wednesday on Instagram.
A source confirmed the news to E!, saying the couple "still really care for each other."
O'Day and Pauly D met on the E! reality series "Famously Single," which will premiere June 14.
The singer assured E! "nothing about [their] relationship was not serious" when she announced the split last week.
"I'm the first girl Pauly has ever loved, as he admits on the show," she said at the time. "He brought me into his life with his daughter and mother, who I respect dearly ... At the end of the day, we tried our best."
O'Day confirmed her romance with Pauly D in February, telling E! she and the reality star had "chemistry" on "Famously Single." She acknowledged they may seem "completely different," but said they're "pretty a like in a lot of ways."
"We connect in a really interesting way," she shared. "I mean, it's definitely something you're gonna want to tune in and watch, we have a very funny relationship ... He's incredibly funny and smart and very talented."
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O'Day came to fame with the girl group Danity Kane and released her first solo EP in 2013.
She and fellow Danity Kane alum Shannon Bex now perform as Dumblonde, while Pauly D is known for "Jersey Shore" and "The Pauly D project."
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