Erik Salaj from Winsoft has a com port component for Delphi XE6 Firemonkey on OSX available. This is the latest com port component in a series of com port components that he has available for Windows, Android, and IOS. The implementation is pretty basic with the demo application that comes with the component reading data using a timer. However, if you need to access a device plugged into the com port on your Mac OSX machine this seems like a really good solution. Additionally, because of his other ComPort components available for the other platforms that Delphi XE6 Firemonkey supports it makes this a good cross platform solution for accessing the Com Port from one codebase. The component has a free trial and it is a commercial component but is reasonably priced. Additionally, all of the components from Winsoft can be had combined into a giant component suite for a really good price.
FMXExpress.com has over 700 tips and tricks for building cross platform apps on Android, IOS, OSX, Windows, and Linux with a single codebase and single UI using the FireMonkey framework found in Delphi and C++Builder.
FMXExpress covers top cross platform secrets for FireMonkey in C++Builder and Delphi XE5, XE6, XE7, XE8, Seattle, Berlin, Tokyo, Rio, and Sydney.