He is a New York Mets fan.
Played drums in the band called "Slow Kids At Play".
Dylan's father is of Irish ancestry. Dylan's mother is of Italian, Spanish, and English descent.
His father is a camera operator and his mother was a stage actress when she was younger.
He first got noticed in his senior year of high school by a director who saw the YouTube videos he did at the time. After the experience of filming a web series with her, he decided to pursue a career in acting while taking classes at community college. He eventually left college after a few months when he scored the role of Stiles on MTV's Teen Wolf after four auditions.
He has one older sister named Julia; they are 16 months apart.
Was reported to be considered for Peter Parker/Spider-Man in Captain America: Civil War (2016) and Spider-Man: Homecoming (2017), among others, before the role went to Tom Holland. On March 1, 2015, a source on Reddit stated that Dylan had been cast in the role, a story that was repeated by most prominent film news sites. In both the 2010 and 2015 casting cycles for Spider-Man, no actor reported by film websites as being all-but-confirmed for the role has ever ended up actually cast as Spider-Man. The other actors erroneously reported to have been cast are Logan Lerman (in April 2010), Jamie Bell (in June 2010), Josh Hutcherson (on June 30, 2010, one day before the official casting of Andrew Garfield), and Asa Butterfield (in May 2015).
Was seriously injured during filming a scene for Maze Runner: The Death Cure (2018). Filming for the film was immediately halted after O'Brien was hospitalized and the release date was pushed back, from February 17, 2017 to, ultimately, January 26, 2018. [March 2016]
Dylan was the drummer for the band Slow Kids At Play.