"Welcome to Nashville, b*****!"
"This is how it's done in TN," Bilson wrote, along with the hashtag "#propergreeting."
Carmack co-starred on The O.C. with Bilson for two years, before he left the show as a full-time regular in 2004.
Last Thursday, Bilson announced her return to television with a photo of Nashville's fictional record label, Highway 65. "Music city, here I come," the 35-year-old actress wrote on Instagram.
She won't be the only familiar face heading to the country music drama. Empire's Kaitlin Doubleday will be joining the ensemble, making the news public with a similarly cryptic Instagram pic of The Bluebird Cafe.
Nashville returns this summer on CMT.
For more on Bilson's new gig, watch the video below.