If you are looking for an application to take notes: Of course there are many alternatives and options. From the simple Windows Editor or the quite respectable “Notes” on MacOS to lots of freeware note-taking programs there is a lot of choices. Especially on mobile devices, the number of note-taking programs is vast. If a cross-device synchronization is needed, there are significantly fewer options. Google Notes (formerly known as Google Keep) is one of them, although not nearly as flexible and powerful as OneNote.
When it comes to that flexibility, power and the general concept of OneNote (although many differences in details and organization) there is only one real competitor: Evernote.