From http://www.barebones.com/support/develop/odbsuite.shtml

[…] an Apple Event-based interface that developers may take advantage of in order to provide better integration between their products and BBEdit

This interface is also used by e.g. Xcode for its external editor and it’s implemented by SubEthaEdit.

This appears to be untrue – Xcode uses the API defined by MetroWerks CodeWarrior

Also check out EditInBBEdit, where you can find a set of classes to add the ODB client side in your cocoa app.