I plan on covering the pond with a plastic liner.
Do I need to have some kind of open hole somewhere for oxygen ?

That depends on your pond volume/depth and fish load as well as amount of
decaying plant material in the pond - it is best to keep a hole open either
by heater or airstone to allow gas exchange - in T.O. where you are (I
believe) you have a colder winter than me in the "sunbelt" (35 mi east of
Windsor) and may need a heater as well. Large fish suffer the most from lack
of o2 and if you have large fish (~12"), I would make sure the pond floor
was as clean as possible and a hole was open with either an airstone or
heater- If you use a heater, you can cut down on the hydro bill by using a
timer or manually plugging it in and unplugging it as needed to maintain a
hole open at least every other day
Gale :~)