Replace keyboard shortcut for ⌥ + d
Create a plain text file with this in it (in TextEdit, use Format &gt; Make Plain Text):

{
    "~d" = ("insertText:", "ð");
}


Call it DefaultKeyBinding.dict (make sure to uncheck “When no extension is provided, use .txt”), and save it in ~/Library/KeyBindings/. (You can press Command ⌘+Shift ⇧+G and paste that path.)

The "~d" means Option-D, and the ("insertText:", "ð") means it will insert a ð character instead of ∂.

You will have to restart your applications in order for this setting to take effect.