Lindsay horsemans' association hosts 4 shows over the summer, they're on the 3rd sunday of every month, 1st one is in may. http://www.lhaweb.ca/index.htm
I'm in oshawa as well.
there is also durham saddle club which has shows in uxbridge. http://www.durhamsaddleclub.com/
Not sure what level you want to show or if you want to do strickly club shows but most of the area fairs have horse show components as well.
I showed at lindsay exhibition last year.