I've no problem with using LibreOffice for most of my document needs, but i haven't found a good substitute for microsoft's OneNote yet. I mainly use it to plan my RPG games and it helps a lot. What alternatives are there for organizing notes on linux, with similar features to those that OneNote provides?
I am surprised that no one has mentioned Rnote yet.
It is my favourite newly-created program for Linux. It is a relatively new app which supports annotating files and taking handwritten notes. You can import PDFs, set the page size to infinite or a fixed size (something OneNote can't do), adjust the background to display grids or lines or dots or nothing with any spacing you like, input text with your keyboard, .... It is available on Flathub for easy installation.
The only major downside is the following: Disclaimer: The file format is still unstable. It might change and break compatibility between versions.
For text-based notes I use Obsidian.
It isn't open source, but it writes standard markdown files to disk, so I can switch programs whenever I like and I am not locked into the Obsidian ecosystem with my notes. That was the main reason why I decided against using Joplin, especially after my experience with converting recipes from Nextcloud Cookbook to markdown ...
In general I am always trying to find a simple file-based solution for whatever I need to do. I want to be able to sync it with Syncthing instead of something fancier that requires a centralised web server or even relies on a cloud service.
Did you know that you can even sync your note using git and thus a git remote server for syncing? It even works with iOS 😃