Ideas for a business policy:
OneDrive and SharePoint are both digital storage areas,
but they serve a different purpose.
OneDrive for Business I compare to a private desk drawer at the office.
A place to store your personal work related stuff in, notes, doodles, ideas, maybe one or two photographs of your family and your dog. Officially it's private, but do know that you're at work and while it's bad form to peek, people might look there, even if it's locked, especially if some kind of calamity happens. So don't hide any secrets in a work related storage space. Work is work.
Things that are strictly home stuff you don't put in a drawer at work,
don't do it either in a digital drawer which is the OneDrive.
Things that belong to home, you do at and keep at home, not at work.
In digital form: store private files in a home related cloud solution
that is your own, disconnected from your work life.
Also don't hide things in a hidden drawer that are needed elsewhere
in the company. In essence, if the private desk drawer and the OneDrive
are thrown away, no essential business data may be lost.
As a policy, people should use OneDrive for things that are handy (or fun),
but in the end can be missed if they accidently get thrown away.
SharePoint is where you put everything else. In physical form it would be the central office cabinet of your department. The important stuff, the notes
and documents that need to be there. Anything that your colleagues might need goes there.
Hope this is helpful.