What documents do I get to prove my ownership of the offshore company?

This depends on the actual configuration of the IBC. If You are a direct shareholder in the company (without using the nominee shareholder service), then Share Certificate(s) and an entry in the Register of Shareholders is Your ultimate proof of ownership. If nominee shareholder is involved, then the clients` proof of beneficial ownership is confirmed by a different legal document. A declaration of trust, a deed of transfer, a nominee services agreement, a beneficial owner declaration can serve as such confirmation. Regardless of the name, such document, signed by the nominees, would confirm in absolute certainty who is the real owner of the offshore company.