Depending on how long you have been living outside Ontario, your OHIP card may not be valid any more (even if the expiry date is not passed yet). In order to keep your OHIP, you should have lived in Ontario for at least 153 days of your first 6 months in Canada and after that at least 6 months a year. Did you do that? If yes, then your OHIP card will still be valid and you should be covered by OHIP for the first 3 months you are living in Alberta.
If you did not spend this amount of time in Ontario, then your OHIP card will not be valid any more. In that case, contact Alberta health using the contact info on this page:
http://www.health.alberta.ca/contact.html Tell them how long you lived in Ontario, how long you have lived outside the country and ask if you move to Alberta now, will your move be treated as an international move and will you get first day health care. If you will get first day health care, then you move to Alberta and register for their health care right away. If they will treat your move as a move from Ontario and you have lost your OHIP already, then you will have to wait for 3 months to get covered in AB.
You can also ask OHIP if you are still covered by using their contact page here:
http://www.health.gov.on.ca/en/common/default.aspx