Похоже, вы здесь новенький. Чтобы принять участие, нажмите одну из кнопок ниже!
A polyfill has already been made for MutationObserver that has been verified to work in Opera 12; you should just be able to copy and paste it in: https://github.com/megawac/MutationObserver.js
I would recommend @require but I don't think old Opera's native UserJS system supports that.
EDIT: Here's my fork: https://greasyfork.org/en/scripts/9243-no-yandex-ads
FTR it seems like the latest Fanboy Ultimate urlfilter.ini already blocked the ad-content, although it did not remove the HTML elements where the ads would go; in my testing, I had to disable the URL Filter File so I could see what I was doing. Also, it turns out that NodeList in Opera can only be iterated via the for-in loop; it must not be array-like, because I got "could not be converted to object" errors with the original code.