The union says nine of 10 bargaining units of workers with the province's community care access centres have voted to strike, with the main issue being wages.
The workers include registered nurses, nurse practitioners, registered practical nurses, social workers, physiotherapists, occupational and speech therapists, among other health professionals.
Health Minister Eric Hoskins says he hopes the parties can come to an agreement and notes the government has increased funding for the community care access centres, including an additional $270 million for the home care sector.
New Democrat health critic France Gelinas also urged both sides to return to the table, saying the best way to protect patient care and respect frontline health-care workers is to reach a negotiated agreement.
The strike affects most areas of Ontario, except for the Ottawa area, central Toronto and communities just west of Toronto, including Mississauga and Halton Region.
However, the union says workers in the Hamilton Niagara Haldimand Brant unit have ratified a new agreement.
The organization that runs the centres says contingency plans are being implemented in all CCACs affected by the strike, to help continue service to patients during the labour disruption.
"The safety and well-being of patients remains the top priority," the Ontario Association of Community Care Access Centres said in a release.
"Patients receiving care in homes, schools and clinics will continue to receive these services as per their individualized care plans," it said. "For example, patients being seen by a visiting nurse, therapist or personal support worker will continue these services without interruption."
It added CCAC staff not represented by the ONA will continue to work.
The union says it has been seeking wage increases equal to the percentages given to the other 57,000 members of ONA in the hospital, public health and long-term care sectors.
It says CCAC members had a two-year wage freeze in their last contract, which expired March 31, 2014.
Union president Linda Haslam-Stroud says the workers provide important care for patients in their homes as well as in schools and other community facilities.
"Government is relying more on community care as it pushes services out of hospitals, and the role that our CCAC members play in the system must be respected," Haslam-Stroud said.
"This strike is about dignity and respect, and also sends an important message that these CCAC CEOs can't give themselves a large salary increase and then unfairly compensate the highly skilled, invaluable frontline workers who provide care."
The CCACs are funded by the provincial government through Local Health Integration Networks.
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