AppleScript support
OmniGraffle has extensive AppleScript support; open the OmniGraffle dictionary in Script Editor for details about OmniGraffle's object model and commands.
You can store AppleScripts in OmniGraffle's scripts folder, for easy access from the Scripts menu. You can also attach scripts to individual objects on a canvas, using the Action inspector. When you activate the Browse tool, then click an object with a script attached, the script runs.
If you select some objects and then choose Copy As AppleScript from the Edit menu, you get the exact lines of AppleScript necessary to create those objects. This is a great way to learn how OmniGraffle's AppleScript support works, or to add specific objects to your own scripts.