While still in college, he earned money working as a laborer for a company building the city's subway system, Metro.

In 1987, O'Sullivan's teaching and computer interests converged when he was appointed an instructor at the West Virginia Laborers' Training Fund.

In that position, he taught training seminars and helped blue-collar laborers learn how to program and operate new, computerized heavy machinery.

[2] In 1993, LIUNA president Arthur Coia appointed O'Sullivan assistant director of the international union's construction, maintenance and service trades department.

During his tenure as assistant director of the department, O'Sullivan convinced the union to build a number of new training centers on the country.

A consent decree permitted Coia (elected in 1992) to remain president so long as he made significant progress toward internal reforms.

[1][5] After being vetted by federal officials, the union's executive council elected O'Sullivan president to replace Coia.

He even co-authored an op-ed piece with United States Secretary of the Interior Gale Norton advocating increased oil production.

Sweeney resigned from the board, then threatened to bring the matter up before a public meeting of the executive council of the AFL-CIO.

O'Sullivan demanded the return of these additional profits, and slowly brought ULLICO back to financial health.

O'Sullivan was particular vocal in demanding that the AFL-CIO rebate up to 50 percent of all dues to international unions for use in new member organizing.

In February 2006, O'Sullivan took the first step toward disaffiliation by withdrawing his union from the Building and Construction Trades Department (BCTD) of the AFL-CIO.

The Laborers and the International Union of Operating Engineers (an AFL-CIO affiliate) quit the BCTD and formed a rival group, the National Construction Alliance.

O'Sullivan made four demands on BCTD: Its leaders must resign and new elections must be held; its budget must be trimmed to permit more money to be spent on organizing; it must alter its proportional representation rules, which give more delegates to smaller unions; and it must revise its rules for determining which unions have jurisdiction over various kinds of work.