You may be deemed "rehabilitated" and enter Canada without applying for rehabilitation by satisfying a border agent that the completion of your sentence for a sole OUI conviction occurred 10 or more years ago.
The process is discussed here. http://www.cic.gc.ca/english/information/inadmissibility/rehabilitation.asp
If you want to apply for "rehabilitation" to prevent being assessed each time you cross the border, the procedure, forms and required documents can be found here http://www.cic.gc.ca/english/information/applications/rehabil.asp
Please feel free to ask any follow up questions.
Thank you for your response and the additional information. Because your concern was getting into Canada with the least amount of trouble, I wanted to look into the possibility of expunging your criminal record in the state of conviction so that no record would be available to Canadian authorities, even though you would still have to answer truthfully if asked about any prior convictions. However, Maine does not expunge or erase criminal records. The only option Maine has is a pardon and these are rarely, if ever, granted for OUI convictions. In addition, it appears a pardon would only change the status of the conviction to non-conviction on your record, so the Canadian authorities would still have access to it. Furthermore, the procedure to obtain a pardon seems to be even more onerous than applying for "rehabilitation" through the Canadian process. The Maine pardon process is described here http://www.maine.gov/corrections/adult/pardon/ Based on the above and my previous answer, your best bet to avoid any future difficulty in crossing into Canada may be to apply for rehabilitation in Canada. The application and information can be found here http://www.cic.gc.ca/english/information/applications/rehabil.asp
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