He serves as a Democratic member for the Hillsborough 10th district of the New Hampshire House of Representatives.
[2] In January 2024 he was elected Vice President of Nashua's board of aldermen.
When Democrats had control of the house during the 2019-2020 biennium O'Brien served as vice chair of the Transportation Committee.
[1][3] In January 2024 John Formella, the attorney general of New Hampshire accused O'Brien, two other aldermen, and the mayor of Nashua of violating the state's anti-electioneering statute.
[7] Formella issued an order instructing the four politicians to cease and desist from violating the law.