Do the individuals-with-significant-control rules apply to private Ontario corporations, public ones, or both?

The individuals-with-significant-control register requirement under the Business Corporations Act is aimed at private corporations, not public ones. If your Ontario corporation is a reporting issuer, or its shares are listed on a designated stock exchange, it's generally exempt from maintaining an ISC register, because public corporations are already subject to a separate, more extensive disclosure regime under securities law that serves a similar transparency purpose in a different way.

For every other Ontario corporation, the overwhelming majority, including closely held family businesses, professional corporations, and holding companies, the ISC register requirement does apply, regardless of how small the corporation is or how few shareholders it has. There's no exemption based on the corporation's size, revenue, or number of shareholders; a small, wholly owned operating company has exactly the same obligation as a larger private corporation with several owners.

This distinction surprises some business owners who assume transparency-register rules are aimed only at larger or more complex corporations. If your corporation isn't a public, exchange-listed company, assume the ISC register requirement applies to you and confirm your register is in place and current.