LOS ANGELES — The December morning that Grammy nominations were announced, Finneas O'Connell woke up early. The 22-year-old had produced and co-written all the tracks on "When We All Fall Asleep, Where Do We Go?" the debut full-length by his sister, Billie Eilish. That album helped establish Eilish, now 18, as one of pop's brightest and biggest new stars in 2019, which also made Finneas one of the most sought-after producers. The idea that at least one of the siblings might be nominated for a Grammy award didn't seem out of reach.

O’Connell wanted to wait until his girlfriend was awake so they could hear the news together, but it was 5:30 a.m. and he felt bad waking her up. His phone was erupting with texts that he refused to look at, but he figured the sheer volume of messages was a good sign. “I figured they wouldn’t text me if it was like, ‘We’ll get ’em next time,’ ” says O’Connell, who goes by Finneas. (Or, sometimes, FINNEAS.)

Eilish wound up with six nominations, and Finneas with five, including album of the year, record and song of the year for the hit “Bad Guy” (which he co-wrote), and producer of the year (non-classical).

A few weeks after that nominations morning, Finneas is sitting in a coffee shop in the Los Feliz section of Los Angeles, the upscale neighborhood where he just bought a house, and he still can’t believe it — even though he can actually kind of believe it. He’s at that place, in the late-early stages of fame, where everything good seems possible and nothing too calamitous or traumatizing or weird has happened. He’s genuinely open and friendly, and appears to go unrecognized by everyone but Shakey Graves, a musician friend he runs into on the street.

In October, Finneas released “Blood Harmony,” a seven-song EP that was mostly cobbled together in dressing rooms and tour bus lounges and hotel rooms when he was on the road with Eilish. It’s a sleek, striking work, poppier and more linear than “When We All Fall Asleep.”

In its own offhand way “Blood Harmony” is more commercial, though it doesn’t need to be. His sister’s success has changed the topography of Finneas’s life in almost every conceivable way. Mostly, it has freed him. He can make any kind of music he wants and never needs to have a hit in his life. He can take or leave the parts of fame he doesn’t want, unlike his sister, who is stuck with all of it.

“She walks around, and because there’s a billboard of her face on Sunset Boulevard, people really recognize her,” he says. “In my perfect world, I get to be a professional musician and still go to Trader Joe’s.” For celebrities in Los Angeles, being able to go to Trader Joe’s is the ultimate dividing line, like still being able to take the subway is for celebrities in New York. Eilish usually needs security to go anywhere.

“I think she does really well with it, and she’s very deserving of the adulation that her supporters give her,” Finneas says. “I’m maybe a little less cut out for that level of white hot, kids-chasing-you-through-an-airport, ‘Hard Day’s Night’-level stuff.”

Finneas grew up in the Highland Park area, where his parents were working actors who taught their children at home (there were two bedrooms; they co-slept). “We were a very crunchy, sort of hippie-dippy family,” Finneas recalls. They were, and are, close. Each member of the family speaks with obvious affection about every other member of the family, especially Finneas for Billie.

“We have never put any kind of emphasis on getting a job and making a living,” says his father, Patrick. “If we modeled anything, it was being broke and artsy.”

Finneas began playing instruments and writing songs when he was very young. It didn’t take long for his parents to realize he was special. “Patrick and I would hear something, and walk into the room and go, ‘Who wrote that?’ ” his mother, Maggie Baird, recalls. “And he’d go, ‘I wrote that.’ Every time. I remember telling a person or two who was a singer or a songwriter, I’d be like, ‘I don’t know what to do, my son has this crazy talent.’ They were kind of like, ‘Oh, yeah, you’re the mom, you can’t possibly know.’ And I’d be like, ‘I think I do know.’ ”

When Finneas was 12, he attended a songwriting class taught by Baird, a musician in her own right. Everything clicked into place. “It’s as if she handed him a songwriting Rubik’s Cube, and he solved it in three seconds,” Patrick says.

Afterward, he began to get serious about making music and eventually formed a pop-rock group called the Slightlys. They performed around town, made an EP and played a local Warped Tour date one summer after winning a battle of the bands contest. Finneas, who was 15, assumed that the show would be the highlight of his life. “Even that one day was so heatstroke-y, and so hectic and stressful that I was like, ‘This thing I thought was my lifelong dream is not my dream at all,’ ” he recalls.

Finneas also did some acting, appearing in a 2013 film, “Life Inside Out,” opposite Baird (the movie’s co-writer) and also on “Modern Family.” He did four episodes of “Glee” during its final season, which he described as feeling like the new kid in class the week school let out. “I definitely felt like I’m part of the end of this seminal thing,” he recalled. “It’s so funny to be doing your fourth episode of a show and everybody’s weeping, and it’s like, their 130th episode.”

When Finneas was 18, he wrote a song for the Slightlys called “Ocean Eyes.” In that early incarnation, “it sounded a lot like Soundgarden,” he says. “It was a big, soaring electric guitar and drum record. It was the wrong outfit for that song.”

“Ocean Eyes” wound up being right for Billie. They worked on the track together — Finneas produced and wrote it, Billie sang it. In November 2015, they posted the song, by then a disassembled, darkly evocative ballad, to SoundCloud and watched as it started to find an audience.

“The best part about it was it was such baby steps, comparatively,” Finneas says. “The first night was a thousand streams, and we were like, ‘Oh my God!’ Then a week later, it was 10,000, two weeks later it was a hundred thousand. They weren’t Bieber YouTube numbers, where you’d put up a Justin Timberlake cover and you’d get 15 million. We appreciated every step of the way.”

Finneas enlisted a manager he knew, and Eilish began talking to record labels. In many stories, this is where things would begin to break apart, where the prospect of money and fame could have ruined everything. If the O’Connells had been friends instead of siblings, they might have fallen out. A romantic couple might have broken up. But Finneas and Eilish, who grew up close and were still stuck living together, had almost no choice but to carry on as a team. Finneas accompanied Eilish to meetings, and they’d talk about it on the way home. They had a management team who was invested in their joint success, which helped. But if they’d had different representation? Finneas doesn’t even want to think about it. “If I’d had separate managers,” he says, “it probably would’ve been a whole thing.”

That a team of A-list writers and producers was not brought in to oversee Finneas — or get rid of him entirely — is remarkable. In a darker timeline, Eilish, having used “Ocean Eyes” to get a label deal, went off to make her debut album with Max Martin, while her brother, the musical Rubik’s Cube-solver, went back to uploading tracks to SoundCloud in his childhood bedroom. “In the alternate reality where I wasn’t involved at all, and I’d been like, just, sweating my way through, trying to have a music career for years? And then my sibling had one and I wasn’t involved at all? I think I’d be very tortured by it,” he says. “But the fact that we’ve had one in tandem makes a lot of sense.”

The main reason their partnership has endured, O’Connell thinks, is that Eilish is uncomfortable with the traditional way pop music gets made: An artist enters the studio with a proven hitmaker, who is usually older, usually male and often someone she doesn’t know. They discuss whatever is going on in her life, and the hitmaker uses these filaments of ideas to weave together a song. Eilish hated this. “Billie doesn’t actually like recording sessions at all,” Finneas says. “We like making music together. She doesn’t like going to some big studio and having them pretend to be a therapist for a couple hours. So by default, we always make the good stuff together.”

Finneas has also begun to ramp up his work for other artists. He co-produced Selena Gomez’s hit “Lose You to Love Me” and some new tracks for Swedish pop singer Tove Lo. He has worked almost exclusively with female artists. “It’s definitely femme heavy on my résumé,” he says. “I think there’s a real vulnerability to the lyrics I’m interested in writing, and I think there are a lot of incredible women in music right now that are willing to be vulnerable, and there’s a little less male vulnerability.”

Finneas worked on several songs on Camila Cabello’s new album, “Romance.” “For the song we wrote together for my album, we clicked instantly as friends during our session,” Cabello says. “When he sent it back to me after the work he did on the production, you could feel his enthusiasm in each paragraph he wrote to me explaining why he’d chosen the sounds he did. He just really cares, and it’s that kind of pure love for what he does that makes his work so special.”

Finneas has been working on a full-length debut album of his own for what seems to him like forever. “Blood Harmony” was hard enough to put together, but an album has to be a statement of purpose, and it’s been slower going than he’d hoped.

He’ll be with Eilish when she tours the world this year, working on the album in his spare moments. While it’s not something he takes for granted, the longer they work together, the more it feels to him like a partnership that could last. Eilish might take a bigger role in the production of her work going forward, he thinks. Everything else they’ll figure out as they go along.

“It’s me being of service to whatever she needs,” he says. “Whenever duty calls, I say, ‘Yep. Let’s go.’ ”