He resumed his testimony just after 9 a.m., and remained on the stand nearly all day.
He spoke of former Illinois House Speaker Mike Madigan's influence in the naming of Juan Ochoa to the ComEd Board.
An email was brought into evidence from Madigan's administrative assistant April Burgos.
It was sent to ComEd CEO Anne Pramaggiore, and included Ochoa's resume.
It said, "Hi Anne, Speaker Madigan asked me to send this to you. Please confirm. Thanks, April."
At the time, Jesse Ruiz was stepping down from the ComEd Board. The board was mainly an advisory board, and the role came with a $78,000-a-year paycheck.
O'Neill, who was in his new role at Exelon at the time, said he was forwarded Ochoa's resume, and a background check began.
O'Neill said he spoke with Pramaggiore sometime in 2018 about his concerns over having someone connected to the speaker on the board, but Pramaggiore said it was important he be selected.
Mike Madigan trial LIVE updates: Former ComEd attorney resumes testimony in Madigan trial
"It seemed to me bad optics to have someone directly connected to the speaker on the board," said O'Neill of his conversation with Pramaggiore, adding that, "she acknowledged that, but she was interested in having Mr. Ochoa on the board. She believed there was a need to maintain good relations."
Pramaggiore, who along with McClain and two others, was convicted last year of attempting to bribe Madigan in exchange for his help getting legislation favorable to ComEd passed, eventually got her way.
Ochoa was the only candidate considered for the job, and, despite it taking over a year, he was named to the board in April of 2019.
O'Neill said he later learned others, including Rahm Emanuel, supported him, as well.
O'Neill was also asked if he thought lobbying was legal and necessary. He said "yes."
Defense attorneys finally got their chance to question O'Neill Tuesday. So far, he has provided the most in-depth look at ComEd's decision to provide contracts and jobs to Madigan's political allies.
Madigan's attorney Daniel Collins tried to raise doubt on the Reyes Kurson legal contract, saying it's possible Mike McClain could have just used Madigan's name instead of working at Madigan's request.
His attorneys appeared to be working Tuesday to convince the jury what McClain did was legal lobbying.
"Was it sometimes your practice to hire someone because one of the things they could bring to the job was their connections to elected officials?" McClain's defense attorney Patrick Cotter asked.
"Sometimes, yes," O'Neill said.
On Monday, O'Neill got to the heart of the corruption charges against Madigan and his co-defendant, McClain.
Prosecutors drew a direct line between ComEd legislation pushes and a contract entered into by ComEd, in October 2011, with Reyes Kurson, a law firm headed up by Victor Reyes, a political ally of Madigan's.
Former Madigan aide Will Cousineau took the stand in the afternoon.
He also testified in the ComEd Four trial about how Madigan wielded power.
The former Illinois House speaker is accused of a sophisticated bribery and racketeering scheme.
The trial could last three months.
Related Coverage: How we got to here
Opening statements begin in former IL House Speaker Mike Madigan corruption trial
Jury selection begins this week in corruption trial of former Illinois House Speaker Mike Madigan
ComEd to pay $200M in federal bribery investigation; Illinois House Speaker Mike Madigan implicated in charge
House Speaker Michael Madigan polling Democratic caucus on whether he should step down
Special House committee to probe Michael Madigan bribery allegations stemming from ComEd investigation
IL House Speaker Michael Madigan continues to lose support amid ComEd investigation implications
Mike Madigan resigns as Democratic Party of Illinois chairman
Former IL House Speaker Mike Madigan indicted on charges including bribery, racketeering
Former IL House speaker Michael Madigan indicted on racketeering, bribery, more
Mike Madigan charged with crimes usually associated with Chicago mob
Michael Madigan indictment: Former House speaker pleads not guilty in federal corruption case
Former Illinois House Speaker Michael Madigan enters not guilty plea on all counts in federal case
All defendants found guilty on all counts in 'ComEd 4' trial surrounding ex-Speaker Mike Madigan
Jury finds Mike Madigan confidant Tim Mapes guilty in perjury case
Former Speaker Mike Madigan lawyers ask judge to dismiss 14 counts, citing recent SCOTUS ruling