Allowing # and & in OneDrive for Business
At Microsoft Ignite 2017 they mentioned that OneDrive does now allow # (hash) and & (ampersand) but it needs to be configured on the tenant. If your current setting is NoPreference meaning Microsoft will roll it out when it feels like it. If you want these characters accepted earlier then it can be set with
Set-SPOTenant -SpecialCharactersStateInFileFolderNames Allowed
You can see your current setting as follows
Connect-SPOService -Url https:\\rnib-admin.sharepoint.com Get-SPOTenant | fl SpecialCharactersStateInFileFolderNames