In the high season in Ontario, is it possible to camp on campsites without early pre-reservations?

From the Ontario Parks website:

Reservations are NOT always necessary at Ontario Parks; even during busy summer weekends. You can reserve a campsite at most parks five months in advance of your date of arrival. Reservations can be made online or by contacting our call centre at 1-888-ONT-PARK. Some of our most popular parks such as Killbear, Sandbanks and Pinery do fill up quickly but many other parks will have sites available. The vacancy report is a great way to search for campsites. A few parks do not accept reservations and you are always assured of getting a campsite when you arrive at these parks.

In addition to the provincial parks there is the possibility of camping on Crown Land such as in Temagami. Because this is generally not accessible and not serviced in any way (no toilets, no running water, just lakes and islands with no buildings) the number of people who want to get in a canoe and get there is generally small enough that finding a site is not a problem. Note that you cannot camp in conservation areas or on private property. Choose a provincial park that offers services you want (for example showers or flush toilets) and is in a convenient location. The further from Toronto you go, the less likely you are to need reservations. Be aware that some parks are practically within city limits.

I have been car camping and canoe camping in provincial parks and on Crown Land for decades. I have often made reservations (sometimes by a phone call that morning before leaving home) and I have arrived without reservations at smaller parks. Never once have I been left without a place to sleep. If you're considering a particular park, ask about it here or on Great Outdoors for more details including how busy it gets.