16 songs you probably didn't know were written or produced by Billie Eilish's brother Finneas

"That girl is so dope," O'Connell told OTW of Evalyn, who included "Filthy Rich" on her 2017 EP "Sandcastle."

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Alice Kristiansen released "Lost My Mind" as a standalone single back in 2017. O'Connell recorded his own version of the song for his 2019 EP "Blood Harmony" — possibly because he still connected to an autobiographical element of the song, since he's cited that as the main reason why he keeps some songs for himself.

"I love working with Gabrielle Current," O'Connell told OTW. "We've written a ton of songs together, she's incredible."

Most notably, O'Connell is featured on Current's 2017 single "Come to Think." You may recognize it from a home video of a young Eilish performing the song alongside her brother.

O'Connell reached out to the band after he heard their cover of "Ocean Eyes."

"Finneas heard it and really liked it and he showed it to Billie and she really liked it and then we ended up working with Finneas," FLAWES told The One in January. "We had like three or four days together, writing songs together. Now, both of them are the biggest things in pop music at the moment."

"But what was crazy is that at that time," they continued, "Finneas was writing the Billie Eilish album and he was telling us all about it and he said 'This record is gonna change the world.'"

O'Connell cowrote and produced the closing track from Wafia's third EP, "VIII."

"We were just hanging out and I started playing the piano in the tiny recording studio we were in, and the chorus that ended up being 'The Ending' came out and we came up with that first line. The song just flew out in the last hour and a half of us being there," O'Connell told Atwood Magazine. "It was a really fast writing process and produced the whole thing around it over the course of the next month."

He also landed a feature credit by providing backing vocals — and if you listen closely, you can also hear Eilish's voice floating in the background, even though she's not featured.

"'Past Life' is a great f---ing song," O'Connell wrote on Instagram when "Past Life" was released. "I knew it the first time I heard it. Lucky to have gotten to produce it! Buckets of love to Trevor and everyone involved in the creation of this track."

"Bruno became one of my favorite artists after his music was recommended to me by my manager Danny back in 2017," O'Connell wrote on Instagram, the same day "The Most Beautiful Thing" was released.

"We met for coffee in the summer of 18, talked for 2 hours, I got horribly sunburnt and we started writing songs whenever we were in the same city," he continued. "Every day I spend with Bruno feels nothing like work and I am so proud of this song we wrote together. Hope you love it."

JP Saxe and Julia Michaels cowrote the ballad, Saxe's third single, and recorded an instrumental demo that made its way to O'Connell.

"He heard the song, the original demo, which was just the piano and our voices," JP Saxe told Front Row's Rob Herrera, "and there was something about the piano vocal that felt really powerful to everyone. And I think Finneas had a reaction to that, and liked it enough that he wanted to accentuate what it was about it that he already liked. And I loved that that was the approach he took."

"When the Party's Over" was the second single Eilish released from her 2019 debut album. It was written and produced entirely by O'Connell.

"I remember hearing it from the other room or something and, like, he was like, 'Billie come here I just wrote this f---ing crazy song,'" she told NME. "I feel like the melodies in that song get you, you know what I'm saying?"

O'Connell also wrote Eilish's "My Strange Addiction," but she contributed production to the song, which was partially inspired by her obsession with "The Office."

O'Connell was just 17 years old when he wrote and produced his sister's debut single.

"In a unique and fantastic instance, everything about the verse happened all at once," he told Ones to Watch. "I sat down at my piano, played and sang the entire first verse at once. The rest of the song took some time to get right but I knew where it was going."

Finneas originally wrote "Ocean Eyes" for his high school band, but he "knew it was meant for Billie" when he heard her sing it.

"She brought life to it that I couldn't believe," he told OTW. "She might be the most convincing singer I've ever heard. I've never doubted a single word she sings. It's such a gift. Her voice is like a Stradivarius violin."

Rebecca Black and O'Connell's girlfriend, Claudia Sulewski, are close friends. Both are YouTubers and have collaborated on videos in the past, some of which have also featured O'Connell.

However, O'Connell cowrote the fourth track on Black's 2017 EP, "RE/BL," before he and Sulewski had met or begun dating. The song was rereleased as a single in 2018.

O'Connell has worked on multiple songs with singer-songwriter Ashe, including all four tracks on her 2019 EP "Moral of the Story: Chapter 1" — but most notably its title track, which recently landed her on Billboard's Emerging Artists chart.

The single, which O'Connell coproduced and cowrote, recently soundtracked a memorable scene in Netflix's "To All the Boys: P.S. I Still Love You." In a pivotal emotional moment, Lana Condor's protagonist Lara Jean Covey breaks the fourth wall and lip-syncs the song directly to camera.

Music supervisor Laura Webb told Variety that she chose the song because "it works really well stylistically, and she's the example of an artist who definitely has a presence but it's a great opportunity to hopefully introduce her to an even bigger fanbase and hope that people love her the way that we all do. It definitely conveys a lot of emotion."

Tove Lo dropped two new tracks in January, just four months after releasing her fourth studio album, "Sunshine Kitty."

Both new tracks, "Bikini Porn" and "Passion and Pain Taste the Same When I'm Weak," were cowritten and produced by O'Connell. He even appears in the music video for the former.

"I've been a fan of Tove Lo since the minute I first heard 'Habits' in 2014," he wrote on Instagram. "Sometimes you can tell instantly that a songwriter or singer is going to have a lasting impact immediately and that's exactly how I've always felt about tove. Writing with her proved me right. Bar for bar, she's brilliant and 'Bikini Porn' was exactly the song I dreamed of producing for her."

O'Connell contributed production to the seventh track on Halsey's newest album, "Manic."

"I couldn't feel luckier," he wrote on Instagram when the album dropped in January. "Halsey's music has inspired me since I first heard 'New Americana' when I was 17."

Camila Cabello teased her collaboration with O'Connell in an interview with Variety and said their song is one of her favorites from her sophomore album.

"Camila Cabello is one of my favorite artists," he wrote on Instagram when the album was released. "She's also a friend of mine now which makes me feel VERY COOL!!! I stood in the crowd at lollapalooza last summer and watched her set in awe. I texted my managers/publisher right then that I would die to write with her."

He added: "I love this album so much and I could NOT be more grateful to be a part of it!"

Selena Gomez's powerful comeback single "Lose You to Love Me" was primarily produced by the Swedish duo Mattman & Robin, but O'Connell is credited with additional production.

Eilish is signed to the same label as Gomez, and O'Connell was offered a chance to add some flourishes to the song when it was nearing completion. He says he added some "textural stuff," like string plucks and overtone synths, to "make the production feel a little bit more alive."

"He just added that final touch, and it just really made a difference from the second chorus into what he did in the bridge," Gomez told Sirius XM. "It was really awesome."