A Microsoft file hosting and synchronization service.
The OneDrive folder is on your C drive and takes up space on the C drive. OneDrive is not on a separate drive. It's a folder.
To avoid OneDrive altogether and never worry about it again, I suggest you create Manila folders at C>Users>username>ManilaFolder and keep your files there. That means not using folders with colored icons but only Manila folders. Would you like to do that?
You can make new manila folders for Documents, Pictures, Music, and Videos and place them in locations as shown below. Desktop is the only folder that you cannot make into a Manila folder. If you keep your files in those folders you will avoid OneDrive completely.
C>Users>username>Documents
C>Users>username>Pictures
C>Users>username>Music
C>Users>username>Videos