If you are barred from reentry because of accrued unlawful presence, then you are barred from reentry, regardless of the country of your citizenship. If you overstay the expiration date of your OPT, you will fall out of status but you will not be accruing unlawful presence since your F-1 I-94 does not have a definitive expiration date but, rather, has D/S (duration of status).  But I would say that a better solution for you, if you know that you will need to stay beyond the expiration date of your OPT is to apply for a B-2 change of status.  That will enable you to have time to finish up all your business in the US prior to immigrating to Canada.