@destroyerx there are some to-do list extensions you can use in the side bar. For example:
https://addons.opera.com/es-419/extensions/details/todoist-sidebarunofficial/
https://addons.opera.com/es-419/extensions/details/taggingnotes/
I am not related with any of them. I use Google Tasks. There is an extension, but it seems broken (I didn`t check it):
https://addons.opera.com/es/extensions/details/gtasks-sidebar/