Formed in Brighton in 1988, the folk-rock group will close the all-day festival taking place in the grounds of Hatfield House on Sunday, July 16.
Currently celebrating the 25th anniversary of their best known album, the group will play second release Levelling the Land at FBTO 2017.
Fronted by singer and guitarist Mark Chadwick, Levellers will be appearing at Folk by the Oak for the first time.
Having announced Kate Rusby last week, the Folk by the Oak team has also added Show of Hands to the 2017 line-up.
Being a one-day festival with two stages in a compact and leafy site, Folk by the Oak organisers are able to create a day packed with wall-to-wall folk and acoustic music.
