OPT expired May 2013. F-1 Status is D/S. Need to go back to Canada but worried about re-entry. Haven't worked even during OPT.

As a Canadian you do not need a visa to enter the US. You are admitted for duration of status and generally do not accrue unlawful presence until either an immigration judge or USCIS has determined that you have violated your status. However, upon your attempt to re-enter the US, a CBP officer may deny you admission because of unlawful presence. Since your unlawful presence can only accrue when it is determined that you have violated status, in my opinion a CBP officer cannot deny you admission for unlawful presence. If that happens, contact an experienced immigration attorney to represent you before CBP why you are not inadmissible for unlawful presence.

You violated your status when you continued to stay after your OPT expired and you continued to live in the country without departing. Because you have gathered significant unlawful presence, it is most likely you would not be allowed back into the country. Unless you have some other status that permitted you to stay, or that your Canadian citizenship offers you some other benefit, coming back into the country may prove difficult.

Although you may not have accrued unlawful presence (thus not subject to the 3/10 year bars), you are out-of-status, which can be considered when determining your admissibility to the U.S.

Either way, you cannot enter the U.S. without first going to the Consulate and seeking a visa to enter the U.S. even if you are Canadian.