OneNote Online is described as ', part of Office Online, is the online version of Microsoft OneNote ' and is a Note-taking Tool in the Office & Productivity category. There are more than 100 alternatives to OneNote Online for a variety of platforms, including Windows, Mac, Android, iPhone and Online / Web-based. Most alternatives are Note-taking Tools and Todo List Managers. The best alternative is Joplin. It's not free, so if you're looking for a free alternative, you could try Standard Notes or Google Keep. Other great apps like OneNote Online are Microsoft OneNote (Freemium), Evernote (Freemium), Simplenote (Free, Open Source) and CherryTree (Free, Open Source).
Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, can be copied, tagged and modified either from the applications directly or from your own text editor.
Zettlr is a supercharged markdown editor that combines many writing editor features and collect them in one application. It aims at speeding up your workflow and help you write without any interruptions.