He achieved further prominence for his lead role in the science fiction Maze Runner trilogy (2014–2018), which led to more film appearances.

[3][4] He grew up in Springfield Township, New Jersey, until the age of twelve, when he and his family moved to Hermosa Beach, California.

[8] After graduating from Mira Costa High School in 2009, he considered pursuing sports broadcasting and possibly working for the New York Mets.

In his senior high school year, a local producer and director approached him about working for a web series.

O'Brien subsequently changed his plans to attend Syracuse University[10] as a sports broadcasting major, to pursue acting.

[11] Before beginning his acting career, O'Brien produced, directed and starred in a number of short comedy films which he released through his personal YouTube channel, "moviekidd826".

[13] O'Brien's acting break came when he landed one of the main roles in MTV's Teen Wolf, a series loosely based on the 1985 film.

Also in August, he was cast in Caddo Lake, originally reported by Discussing Film[34] and later confirmed by Deadline Hollywood in October 2021.