A Microsoft file hosting and synchronization service.
If you have set up both accounts on your computer, sign in to the account that has the files you want to move. Your OneDrive files are located in your OneDrive folder.
If they show up as blue cloud icons, set them to "Always available". See this: https://www.youtube.com/watch?v=UzIA9uykIhE&t=43s
Copy the files you want to move to an external drive and eject the drive. Now, sign in to your other account on your computer and copy the files from the external drive to OneDrive.
If you want to transfer files from a business account to a personal account or vice versa, it's easy. See this: https://youtu.be/iRQWD0rdRTw?t=19