Hi, JohnnyBoy, and first of all welcome to the boards, and of course to this great game!
(Although you've been a long time lurker.)
Oolite 1.65 is the latest official
release. Oolite is under continuous development, so there have been a couple of test releases since. Right now we're at version 1.71.1. It's a pretty good release, and it will be replaced with the even better 1.71.2 hopefully soon.
Have a look at the release announcement-thread
in this very Mac-forum for the download link of 1.71.1. I would recommend you try to install it and look how it works on your system.
NB: If you download the full installer, you can install 1.71.1 (or any other version) in a completely different folder than 1.65 and use both. You just have to copy and paste your saved games, AddOns-folder and any other stuff you like from the 1.65-folder to the new 1.71.1-folder. (Almost) everything is usable in all versions, save-games anyhow, and if an OXP requires a specific version of the game, it will say so in its ReadMe, and probably point you to an alternative download that can be used with your version of the game. And you will find these informations here on the board as well.