Preparations

Firmware 

The firmware for Ornament and Crime is on GitHub.

https://github.com/mxmxmx/O_C

Opened a terminal window, cd to your personal projects directory, and clone the project like this:

git clone https://github.com/mxmxmx/O_C.git

Now you have a new directory called O_C. Later, we'll create a fork of the O_C project on GitHub to manage our own changes.

Arduino

I already had Arduino IDE, but I updated it to the latest version here:

https://www.arduino.cc/en/Main/Software

You'll want the latest version (1.8.5 as of this writing) because the Teensyduino plugin, which is necessary to get firmware to O_C, will only install to certain versions of Arduino IDE.

Update 6/28: Disregard the above. Currently, you'll want Arduino 1.8.1 and Teensyduino 1.35 to ensure maximum stability with O_C. 

Arduino IDE comes in a .zip file, not a .dmg file, and there's no installer. So after you download and unzip the file, drag the Arduino icon to your Applications folder.

Teensyduino

After you download, install, and run Arduino IDE, get the Teensyduino plugin here:

https://www.pjrc.com/teensy/td_download.html

To install Teensyduino, just click on the DMG file and follow the instructions. If all goes well, you'll see the Teensy board managers under Tools > Boards in the Arduino application.