Does anyone know if you'd be denied coverage for something that came up during your 3 month wait? Like if you got pregnant a month before your OHIP kicked in. Would they deny coverage for the pregnancy as a 'pre-existing condition'?
No, Canadian health care doesn't do pre-existing conditions. You do your waiting time and get covered and then you are covered for exactly the same things as everybody else.
In Quebec there are certain health care services such as: services related to pregnancy or required by victims of violence, as well as services for individuals suffering from an infectious disease that could affect public health may be provided free of charge during the waiting period. Check this thing out in Ontario, maybe they have the same policy there.