How can I determine which App is Hijacking / Disabling my Option-Enter combo?
The problem was an extra keyboard that I had enabled: Unicode Hex Input. I had definitely forgotten about it. I would do something (not sure what, don't really care) to switch to this keyboard and when this keyboard is active the Option-Enter does not work.

I removed this keyboard from my list of Input Sources: System Preferences -> Keyboard, Input Sources Tab. Problem solved, no longer able to switch to this.

Below is a screenshot of my Keyboard Preference Pane on the Input Sources Tab.


Note: The oval is where the Unicode source was. When there are multiple sources the minus button (bottom left arrow) is active to remove the currently selected source. Also I have Show Input menu in menu bar checked - there will be no menu bar item if you have only one source.