O'Neal, an emergency room doctor, broke off an engagement with the suspect in September, said Chicago Police Superintendent Eddie Jackson.
Hospital officials said O'Neal, a two-year Mercy resident and 2016 University of Illinois College of Medicine grad, was deeply religious and loved going to her church, where she led the choir, according to the Chicago Tribune.
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"That was her one thing she wanted ... to be able to go to church on Sunday," Emergency Department director Patrick Connor said at an emotional news conference.
Some colleagues paid tribute to O'Neal on Twitter.
"I knew her, trained with her, saved lives with her and tonight, tried to save her life," Dr. John Purakal wrote. "Tonight, I broke down in front of my coworkers when we lost her, and tonight I held hands with her mother in prayer. Tonight, we lost a beautiful, resilient, passionate doc. Keep singing, TO."
Officer Samuel Jimenez
Chicago police Officer Samuel Jimenez, a married father of three, had been with the force less than two years before he was shot and killed in the Mercy Hospital lobby Monday, authorities said.
Jimenez, 28, did not have to respond to the deadly shooting at Mercy Hospital because it was out of his district, but he did anyway, according to police Superintendent Eddie Jackson.
"He was the real police. He wasn't in it just for the paycheck," an officer who worked with him told the Chicago Sun-Times.
Sgt. Larry Snelling, who works at the Chicago police academy, remembered Jimenez as a "stellar recruit" who was "on top of his game," according to the Chicago Tribune.
"He was one of the guys we didn’t have to worry about at all," Snelling said. "Just a great kid."
Dayna Less, 25, was shot and killed Monday as she exited an elevator at Mercy Hospital, according to the Chicago Tribune.
The first-year pharmacy resident, who graduated from Purdue University earlier this year, had been working at the hospital since July, the newspaper reported.
Less planned to leave Chicago for Indiana on Tuesday to spend the Thanksgiving holiday with her family, WLS-TV reported. She was engaged to be married next summer, according to WTTV.