While I am not on an OA I can tell you for sure that the medical and police check will be needed each time you get the OA.
However it is not every year but about every 2 years that you need a new OA visa.
The OA visa is valid for 1 year and multiple entry and each time you enter you get a 1 year permission to stay stamp. So you make your last exit and entry just before the visa expires and get a 1 year permission to stay stamp, then if you want to leave and return during that year you also get a re-entry permit. So almost 2 years on that visa