QLD May we sue the bank ourself?

Quite likely, unless you signed something saying otherwise.

Consulting a lawyer is a good first step. Getting a second opinion is even better.

Decide what to do after talking with the lawyer/s. It is not normally a trivial exercise taking on a bank but if you do, be prepared for an appeal if you win the first round.

By default, a consumer can take legal action if they disagree with a decision of either FOS or CIO - the scheme member (ie the bank) is bound by it. So you certainly can take action against the bank. However, I do suggest you get some detailed advice before doing so. FOS, unlike a court, is not bound by the rules of evidence, and can be more flexible in their decision making. Any advice should include a thorough dissection of their decision and why it may be deficient.

Are you able to give a short overview of the nature of your complaint?