The Convention binds its parties to minimum principles on freedom of information. Parties must guarantee everyone to official documents held by public authorities on request. It lays out what is acceptable as a withholding ground, and requires all withholding grounds to be balanced against the public interest. It also requires parties to treat requesters equally, permit anonymous requests, and limits charges to actual costs of reproduction and delivery (so no charging for decision-making time). All of this is backed by public reporting and monitoring by a group of specialists.
How does this compare to our Official Information Act? Firstly, it is narrower, applying only to "documents" (defined as "information recorded in any form", so including digital materials and audio recordings) rather than "information" (which includes things which are unrecorded - such as oral briefings and memories in the heads of public servants). Our list of withholding grounds is broadly compatible, with only 9(2)(k) (to prevent improper gain or advantage) not obviously covered. However, the general public interest test would mean significant changes to the ways we handle requests currently subject to withholding under s6. The reason its worth looking at is because the Convention is technically open to non-European parties by invitation, and formal, regular, international scrutiny of our official information regime is likely to be beneficial, in exactly the same way that it is for human rights. Now that it is entering into force, I think it would be worth the government seeing if they could obtain such an invitation, in order to subject themselves to this scrutiny.
Update: clarified the difference between "documents" and "information".