Dennis Michael O'Brien (born 1952) is an American Republican Party politician who served as the 137th Speaker of the Pennsylvania House of Representatives from 2007 to 2008.

A graduate of Archbishop Ryan High School,[2][3] he attended La Salle University, where he earned a bachelor's degree in Labor Relations.

[8] He worked with Governor Ed Rendell to organize a Bureau of Autism Services within the state's Office of Developmental Programs.

Despite a one-seat Democratic majority, the Democratic leader, Bill DeWeese, was unable to garner the votes necessary to win back the Speakership due to some dissatisfaction within his own caucus because of his handling of matters as leader, and notably due to the decision by one member in his caucus to vote for John Perzel, the incumbent Speaker.

In addition to those responsibilities, O'Brien worked with the Department of Public Welfare to ensure implementation of Act 62 (mandating autism insurance in Pennsylvania) which he wrote and passed while he was the Speaker.

On May 17, 2011, in spite of not being supported by any of the party organizations,[citation needed] O'Brien won one of the five GOP nominations for the City Council's at-large seats, with 17.32% of the vote.

Oh won 18.50% of the vote, being first among the field of candidates, while Rizzo was soundly defeated, coming in 7th out of nine candidates—a result some have attributed to his involvement in DROP, the Deferred Retirement Option Plan.