hi,
I just spoke two days ago with someone in Ontario health services, me being out of the country for more than a year. she said I have to apply for vacation absence, so my coverage does not expire. I had to download a form, fill it in and send it to Ohip absences in Thunder Bay. That letter/request will allow me to have the coverage extended while out of Ontario, with certain conditions (that I have a residency in On, and I have stayed more than 5 months in the previous two years before leaving Canada/On, conditions I do meet).
Not sure what is your particular situation, just call them and explain. They will guide you. I have waited on hold only 15 min.