Who is allowed to bring an oppression remedy application under Ontario corporate law?

Ontario's Business Corporations Act defines who qualifies as a "complainant" eligible to bring an oppression application, and it's broader than just current shareholders. It includes current and former registered shareholders, current and former directors and officers, and any other person a court decides is a proper person to bring the application in the circumstances - a category that can extend to creditors or other stakeholders in appropriate cases, though shareholders and directors bring the large majority of these claims.

Being a complainant only gets you through the door; you still need to show the conduct you're complaining about was actually oppressive, unfairly prejudicial, or unfairly disregarded your interests as that particular kind of stakeholder. A former shareholder, for example, generally needs to point to conduct connected to the period when they held shares or to their departure itself, not simply express general dissatisfaction after the fact. If you're unsure whether your relationship to the corporation qualifies you to bring a claim, that threshold question is worth assessing early, since it can be contested before the substance of the oppression allegation is even reached.