"oh so true" is correct and usable in written English, typically as an informal reaction to something or to emphasize the truth of a statement.

This phrase is a casual way to strongly agree with someone or acknowledge that something is accurate. It’s often used in daily conversations and on social media when you find someone’s statement relatable or right.

We use “so true” when we want to express agreement with something someone has said. We use this phrase especially when we want to emphasize the importance of someone else’s statement.