[1][3] O'Brien resigned from Local 25 and Joint Council 10 on March 1, 2022, in order to serve full-time as General President of the International Brotherhood of Teamsters.

In the fifteen-year span of his presidency, the local's membership increased by 30%, notably organizing a group of more than 1,000 parking lot attendants, many of whom were immigrants from East Africa.

[7] In 2013, O'Brien was suspended for two weeks for threatening members of the reform group Teamsters for a Democratic Union (TDU) who were opposing one of his allies.

[2] In 2017, O'Brien was the lead negotiator for the Teamsters with United Parcel Service (UPS) during bargaining for a new national contract, covering approximately 240,000 drivers, package sorters, loaders and clerks.

[16] On July 15, 2024, O'Brien became the first leader of the IBT ever to speak at a Republican National Convention, where he gave a speech encouraging delegates to respect American labor in spite of differing political allegiances.

[17] Despite historical affiliations with the Democratic Party, the Teamsters decided not to endorse a candidate in the 2024 presidential election after an internal poll revealed that 60% of their members supported Trump.