Sisters Lennon and Maisy Stella as well as "The O.C.'s" Chris Carmack have been upped ahead of season two of the ABC musical.

ABC's Nashville is adding three full-time residents.

Sisters Lennon and Maisy Stella and The O.C.'s Chris Carmack have all been promoted to series regular for the upcoming second season, The Hollywood Reporter has learned exclusively.

Carmack joined the country music drama series for the final six episodes of its freshman run as Will, the friend and roommate to Scarlett (Clare Bowen) and Gunnar (Sam Palladio). While first seen as a potential threat to the new couple, the character later revealed himself to be gay and using a womanizing personality to cover his sexuality so it wouldn't hamper his hopes at becoming the next big thing in country music.

PHOTOS: Catfights and Country From the Set of 'Nashville'

The Stella Sisters, meanwhile, have been with the series since the pilot. While initially cast to play Rayna (Connie Britton) and Teddy's (Eric Close) daughters, visa issues prevented them from appearing in the original pilot. The episode was later reshot after the YouTube phenoms made it to Nashville, where the series films. Nashville marks the first acting gig for the young siblings, whose YouTube videos have millions of views.

Lennon and Maisy's Nashville songs -- including a cover of "Ho Hey" -- are among the show's most popular tracks on iTunes.

In the finale, Maddie (Lennon Stella) was revealed to be the daughter of Rayna and Deacon (Charles Esten), paving the way for a rich story in season two.

The news comes as Nashville heads into its second season without executive music producer T Bone Burnett, whose busy schedule prevented him from returning to the drama series his wife, Callie Khouri, created.

Nashville returns in the fall on ABC. An exact premiere date has not yet been announced.

E-mail: Lesley.Goldberg@THR.com

Twitter: @Snoodit