He is best known for being the lead vocalist, guitarist and pianist[2] of American rock band The Maine which he joined in 2007.

[15] O'Callaghan co-wrote and provided additional vocals for the song "Long Way Home" on Nick Santino's debut studio album, Big Skies.

[16] In 2016, he began a solo project under the moniker John the Ghost releasing his first single titled, "Sour Grapes" on April 12, 2016.

[17] In 2020, O'Callaghan returned to the solo project and released his comeback single "Rolled Down Window".

[21] In 2017, he was nominated at the Alternative Press Music Awards for "Best Vocalist" but lost to Lynn Gunn of Pvris.

[24] He also released a book of poetry titled Sincerely, John The Ghost in 2016 to accompany his solo EP of the same name.

Having the experience of being able to leave Arizona and see other parts of the world made me really appreciate what I have here — the sunsets, the weather and, of course, family and friends.