She extensively documented Chelsea Manning's court-martial, and has researched and reported on topics including WikiLeaks' leak of United States diplomatic cables and Guantanamo Bay files, the war on terror, and the Arab Spring.

[6] From mid-December 2011 until summer 2013, O'Brien created an extensive archive of the only available pretrial transcripts of the court-martial of accused WikiLeaks source Chelsea Manning.

[11] Also in 2015, O'Brien collaborated with William Arkin to publish a two-part Vice News series about American universities with close ties to the military.

[16] In his book Thank You, Anarchy: Notes from the Occupy Apocalypse, author Nathan Schneider relates that during the early stages of OWS, O'Brien's "press releases and tweets became so ubiquitous that people started referring to #OCCUPYWALLSTREET and US Day of Rage interchangeably.

"[18] In 2012, O'Brien was a plaintiff in Hedges v. Obama, a lawsuit challenging the National Defense Authorization Act for Fiscal Year 2012, which allowed the U.S. government to indefinitely detain people who are a part of or "substantially support" Al Qaeda, the Taliban, or similar groups.