@Titsubishi So, it looks like you have to get an Arch maintainer for "Extra" in the official repository to repackage Opera and include it. Looks like it's not something Opera can do itself.
Looks like https://archlinux.org/people/package-maintainers/ are the current maintainers of apps. Guess you can find one and point them to https://aur.archlinux.org/packages/opera and https://aur.archlinux.org/packages/opera-gx for examples of Opera packaging working at least.
However, Chrome is not even in the official repo due to proprietary licensing. I'm guessing the same thing would apply to Opera, but not sure. There's a good chance Opera is only allowed in the AUR.