Why have option- shortcuts in text fields started inserting special characters?
Using option (alt) key is the default way to enter non-latin characters from a keyboard which layout does not have these characters. For instance: I'm Polish, so I need to use Polish characters (like 'ą', 'ę', 'ó' etc.). To do that I use option key (option+a, option+e, option+o). 

You can see the variants of keys of your keyboard by enabling keyboard preview: go to System Preferences, choose Keyboard and check the check box in the bottom called 'Show Keyboard &amp; Character Viewers in menu bar' (http://osxdaily.com/2013/06/21/mac-virtual-keyboard-os-x/). Enable it by opening the keyboard menu ('Show keyboard viewer', you can find the instructions under the same link). Now hold the option key - you'll see the variants of keys in your keyboard - you should see the reason to type 'Ø' when you use option+shift+o.

EDIT:

The answer I wrote was the answer to the question in the title: "Why have option- shortcuts in text fields started inserting special characters?".

There a few options: 


if you could do that in previous version of OS X then the new version may have changed some settings (see: https://developer.apple.com/library/mac/documentation/UserExperience/Conceptual/OSXHIGuidelines/Keyboard.html - they can choose to change keyboard behavior as they want and they mention it). 
Also, since entering the text is system-based functionality it may
not be overriden by default - you need an external tweaking software
to do that. 
It may also be a bug - you're using an unstable version
of the system.


You may find a solution (the tweaking software I mentioned) to your problem in answer of this question:
https://stackoverflow.com/questions/11876485/how-to-disable-typing-special-characters-when-pressing-option-key-in-mac-os-x