Is there a React Development plugin for Opera?

You can install "Install chrome extensions" and then go to React Developer Tools page in chrome store and you'll see "Add to Opera" button

From the react-devtools README on github:

It exists both as a browser extension (for Chrome and Firefox), and as a standalone app (works with other environments including Safari, IE, and React Native).

Note that if you want to run it as an addon in Opera, you can run Chrome extensions in Opera. See: https://addons.opera.com/en/extensions/details/install-chrome-extensions/