Restrict users from saving passwords in One Drive

One possible way you can do is if you have enough sample of those data, then you can train identifier through DLP to look for those files.  Or, you can try to regex to your heart content but most likely you will also get a lot of false positive. 

So all in all, it's a culture issue and should be dealt with by having policy in place, and HR enforcing disciplinary actions.

I was in similar shoe few years ago and pushed password manager - result was no one use it.  We sit down with individuals, yes it took a looooong time, and help them move all their password to password manager, then delete the file in front of them.  We then, audit their data and see if they hide it somewhere else.

No easy, tech only, solution for a culture issue.