If the person you are looking to sue normally lives in Ontario but is away, for instance, they took up a job in Montreal, you can serve the claim on the person in Montreal just as you would if they were residing in Ontario.
If the person lives or carries on business outside of Ontario, you could serve your claim on the person outside of Ontario just as you would if they lived or carried on business in Ontario. If you request it, the court may award additional costs to you to cover any extra expense involved in serving the claim outside Ontario.