Detective Allan Reddins, 40, of Chicago responded about 9:30 a.m. to the 1000-block of Lake Street for a report of a person seen leaving a bank in the area with a gun.
At some point, shots were fired in the 800-block of Lake, police said.
Reddins was shot in the left side, and was taken to Loyola University Medical Center in critical condition, police said.
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He died about 10:10 a.m.
The male suspect was shot in the leg, police said. He is in custody, and being treated at Loyola.
He remains stable, police said.
It was not clear what led up to the shooting.
Police have blocked off the area.
Reddins' body was brought to the Cook County Medical Examiner's Office in a procession from Loyola about 3 p.m.
Body of fallen Oak Park detective brought to medical examiner in procession
Reddins' death marks the first line-of-duty death for the Oak Park Police Department since 1938, the department said.
He was sworn in as an officer in May 2019.
"The Village of Oak Park extends its deepest condolences to Detective Reddins' family, friends and colleagues within the Oak Park Police Department and the wider law enforcement community as they process the grief caused by this senseless violence," police said in a news release.
The investigation is ongoing.
Oak Park police are expected to speak during a news conference at 4:30 p.m.
Anyone with information is asked to call Oak Park police at 708-386-3800.
Anonymous tips can be provided at 708-434-1636 or online at www.oak-park.us/crimetip.
In a statement Friday, Chicago police said, "The Chicago Police Department extends our heartfelt condolences to the family, friends, and coworkers of Oak Park Police Department Detective Allan Reddins who was shot and killed in the line of duty today. We will continue to assist and support our neighboring department through this difficult time."
The Chicago Fire Department said, "The Chicago Fire Department shares our deepest condolences with the Village of Oak Park on the LODD of Police Detective Allan Reddins. We stand in solidarity with our brothers and sisters of the Oak Park Police Department.
May he rest in peace."