O365 Systems must not allow the re-use of the last 10 Passwords?

[Marvin Oco]

O365 Systems must not allow the re-use of the last 10 Passwords.

where to set this?

[#2]

AFAIK, this cant be set! By default the same can’t be used during a password change but there’s no check against the history!
I’d recommend you read this documentation as well:

https://docs.microsoft.com/en-us/office365/admin/misc/password-policy-recommendations?view=o365-worl...

[#3]

If you use AAD Connect to sync your users from on-premises Active Directory to Office 365, you would be able to enforce this using on-premises AD - because password changes are managed there.

[#4]

Yeah, keep in mind most of my answer regarding passwords in your posts are referring to cloud only!