What's the difference between suing for oppression and suing for breach of a shareholder agreement in Ontario?

A breach of shareholder agreement claim is a contract claim: it asks whether a specific written term was violated, and remedies are generally tied to what the contract itself provides for or to ordinary contract damages. An oppression claim is broader and doesn't require a written agreement at all - it asks whether the corporation's conduct was oppressive, unfairly prejudicial, or unfairly disregarded your reasonable expectations as a shareholder, which can include expectations that were never reduced to writing.

The two often overlap in practice: conduct that breaches a shareholder agreement is very often also oppressive, since violating an agreed term usually defeats a reasonable expectation you had as a shareholder. Where they diverge is scope and remedy - oppression gives courts much broader remedial flexibility, including a buyout, restructuring corporate decisions, or other tailored relief, while a straight contract claim is generally more limited to what the agreement itself contemplates or to damages. Many claims plead both, using the contract claim where the written terms are clear and the oppression claim to capture broader unfairness the contract doesn't fully address.