http://www.health.gov.on.ca/english/public/pub/ohip/longer.html said:
Absences Outside Canada
You may be eligible for continuous OHIP coverage during a longer absence when you are away from Ontario for one of the following reasons :
Study full-time outside of Canada
Work outside of Canada
Charitable work outside of Canada
To be eligible for continuous OHIP coverage during one of these absences, you must first meet certain physical presence requirements. You must be physically present in Ontario for at least 153 days in the 12-month periods for 2 consecutive years immediately before the absence. Different requirements may apply if you are an Ontario student studying full-time outside of Ontario but within Canada, and you choose to continue your studies at an educational institution outside of Canada.
To confirm your continuous OHIP eligibility during any longer absence from Ontario, you should contact your local Service Ontario – Health Card Services – OHIP office before you leave the province. You will need to show a document explaining the reason for your absence as required (for example, a letter from your school, employer or sponsoring charitable organization).