Cheap last minute flight from Toronto to Ottawa?

Not the slightest bit likely. Also see e.g. Why don't airlines offer last minute empty seats cheap?

Airfares of major airlines are meticulously calculated to maximize revenue, and are published some time in advance. Price discrimination would break down if they allowed their frontline agents to negotiate fares, or set them.

They can be so meticulously calculated because airlines have enough historical data on a route like YYZ-YOW to know approximately how many people will fly and what they are willing to pay, and will have adjusted their capacity and fares over time to suit. Any seats that are still available at the last minute are for the occasional walk-up purchaser, who is typically a business traveler whose is price-insensitive because his or her employer is covering the cost of the ticket.

The best that an agent can do on the day of the flight is search for the best available fare. It is theoretically possible that they can find lower fares in the airline's reservations system that are not available or no longer available from the website or from third parties. But for the reasons already mentioned, it's almost assured that only the most expensive walk-up fares will remain available for booking on the day of departure.