For O'Brien, 54, the surprise election presented an opportunity to respond to voter demands for reform and usher in a new spirit of bipartisanship to a traditionally divisive arena.
But many Republicans, who backed incumbent Speaker Rep. John Perzel of Philadelphia, feel O'Brien's deal with the Democrats - and more recently his left-leaning voting record - is a betrayal of his party.
If the Republican caucus were a Survivor episode, they say, its leaders would likely vote him off the island.
When asked last week whether O'Brien was now a man without a caucus, House Minority Leader Sam Smith said unequivocally: "Yes."
For his part, O'Brien says he's ignoring his critics and is focusing on bringing openness to Harrisburg and civility to the political debate.
"A few people have reservations, but I'm not here to make people happy," O'Brien said in an interview last week. "I'm not deterred by it."
In five months, he has led an energetic reform movement that has transformed the internal workings of the House, eliminating last-minute amendments to bills, requiring posting of amendments, and ending late-night sessions.
The Speaker's Commission on Legislative Reform also recommended establishing limits on campaign donations and expanding public access to government records. O'Brien says he is committed to seeing those measures become law.
"None of these things would have happened without O'Brien," said House Majority Leader Bill DeWeese (D., Greene). "It has altered the political landscape substantially."
O'Brien has launched a novel experiment in legislative leadership. Among his first orders of business, O'Brien assembled a bipartisan staff and formed the Speaker's Reform Commission, charged with overhauling the way the House conducts business.
At the same time O'Brien has continued his vocal advocacy for his most important cause: improving the lives of the disabled, particularly children with autism.
At his urging the House quietly adopted closed-circuit captioning for the first time, so the hearing impaired can now follow the proceedings.
But lingering animosity among Republicans has made him an outcast in some party circles.
What really ticked off Smith (R., Jefferson) and other Republicans was O'Brien's support of Gov. Rendell's budget earlier this month. He was the only Republican to support it.
"Since his election, the speaker has voted extensively with the Democrats, Smith said. "Dennis wants to be in the middle, but at the end of the day you have to be either one or the other."
O'Brien said it was a "test of mettle" that he did not regret.
"I moved the ball forward; that was the important thing," he said.
Supporters say O'Brien, who is not known as a fiscal conservative, was voting his conscience as he had done in the past, and they blamed Republicans for their lingering anger over losing the speaker vote.
"Some seem to want to continue the battle of Jan. 2," said Rep. Curt Schroder (R., Chester), who voted for O'Brien as speaker and is a member of the Reform Commission.
"The budget vote didn't surprise many of us," he said. "It's the same vote he would have made whether he was speaker or a member of the caucus."
O'Brien's election was a master stroke by up-and-coming State Rep. Josh Shapiro (D., Montgomery) with the support of DeWeese, who had hoped to return to the speaker's post he gave up in 1994. But DeWeese, knowing that with the Democrats holding only a 102-101 majority he couldn't win the speaker's race, supported O'Brien's bid. In the end, six Republicans unhappy with Perzel's leadership voted to support O'Brien.
"With his affability, broad-beamed Irish smile, and Niagara-like shock of hair, he is the perfect manifestation of the new chamber," DeWeese said. "Under Speaker Perzel's stewardship, it would be a reversion to the status quo."
A Perzel spokesman did not return a phone call seeking comment.
During a recent session, O'Brien adjourned the chamber for the day and sped off to welcome lawmakers and lobbyists participating in the speaker's annual scholarship golf tournament 15 miles away in Hershey.
In an interview in a lounge at the Hershey Country Club, O'Brien said he was operating the same way he always had: as an independent-minded lawmaker who represents a diverse district.
He views his speaker's role as a bridge builder among caucuses that operate more like autonomous continents.
"It's a struggle for ideas; it's a struggle for priorities," O'Brien said, speaking in his trademark animated style, hands sweeping the air for emphasis. "There is a high level of civility that is attached to this conversation."
But some Republicans, like Smith, say O'Brien is seeking an unreachable middle ground in an arena where party loyalty is paramount.
Still, Smith says he's not yet ready to remove O'Brien's place setting at the caucus table.
"The level of warmth may be tempered from the past years, but we haven't closed the door," he said.
O'Brien has maintained he will stick with his party despite DeWeese's concerted efforts to persuade him to join the Democrats.
But whether O'Brien can sustain the goodwill through the next election is uncertain. Berwood Yost, a pollster from Franklin and Marshall College, said that if Democrats build a more solid majority, O'Brien is "expendable."
"There was a risk and a reward," Yost said. "It was an interesting political calculation, but he's obviously a short-term speaker."
DeWeese last week said he planned to run for speaker in 2009, but also said he'd like O'Brien to be part of his leadership team.
Some Republican supporters say they are more concerned about the lessons learned from November than about O'Brien's perceived party affiliation or voting record.
"In the last election cycle, voters said, 'When you get to Washington, get to Harrisburg, give us meaningful, effective legislation,' " said Rep. Kerry Benninghoff (R., Centre), who voted for O'Brien for speaker. "They don't care who gets the credit or who sits in what chair."
Contact staff writer Amy Worden at 717-783-2584 or firstname.lastname@example.org.