I've seen a way to declare a specific COM port on Windows so the Arduino is always read at the same port, but is there a way of doing that on OSX?
The machine I'm running my project on needs the arduino to be on the same port every time it restarts and sometimes it'll get set to a different port.
Depends on the USB device.
With FTDI, it's 100% consistent. The FTDI has a serial number in the hardware and a well written drive that makes this possible.
With the newer USB-CDC devices (e.g. arduinos that usa an Atmel chip as the USB interface), that's not possible since there is no serial number (or other unique identifier). Technically the driver could create a device special file name based on the location in the USB tree (e.g. KeySpan on OS X does this), but Apple didn't put that functionality in their USB CDC driver.
-j