The slain police chief was identified as Steven Eric Disario, who headed the Kirkersville Police Department, Licking County Sheriff Randy Thorp said.
Officials say Hartless killed two female employees-Marlina Medrano, a nurse, and 48-year-old Cindy Krantzat, a nurse's aide-at the nursing home in Licking County.
The authorities have not given a motive for the shooting, and have not identified the other victims. As deputies cleared the building, residents and employees were found barricaded in several places, Thorp said.
A woman who lives about a block from where an OH police chief and two nursing home employees were killed says she heard shots fired following by screaming and then sirens. The village of about 500 residents is roughly 25 miles (39 kilometers) east of Columbus.
Authorities say a gunman shot Kirkersville Police Chief Steven Eric Disario, who'd only been on the job for about three weeks.
He had six children, and one more "on the way", Mr Thorp said, adding that "it's a real hard day for all of us".
Disario was responding to a report of a man with a gun, and his last radio communication said he had the suspect in sight.
Two employees of the centre and the suspected gunman were found dead inside the nursing home, the sheriff said. Pine Kirk owner Tom Rosser said they were taken to other area nursing homes temporarily.More news: Puerto Rico requests bankruptcy protection for public debt
The chief of the Kirkersville Police Department was killed in a shooting Friday morning. Disario was shot at by the suspect immediately upon entering the nursing home.
The nursing home appears to be located in a residential area, and people in nearby houses were told by law enforcement to stay inside while authorities handled the situation.
A woman who works about a block away said she heard gunshots followed by screaming and then sirens. "You see these types of things happen all over the country".
As of May 3, the facility was serving 23 Medicaid patients, according to the Ohio Department of Health. "They provide them with a small environment that's less institutional than some facilities might be".
Kirkersville Elementary was placed on lockdown and its students transported to the town's middle school.
The Ohio Bureau of Criminal Investigation is handling the case.
Kasich ordered flags flown at half-staff at the Statehouse and at all public buildings and grounds throughout Licking County in central OH, where the shootings happened.