Could you please let us know what all information you need in the report. we can prepare a powershell script and shae with you. Incase if User Name, Display Name and Email address will help you, you can use below PowerShell scrip and run on Exchange online powershell to get the output-
Well you have the ImmutableID/SourceAnchor parameter, it's a 1:1 mapping between cloud and on-prem objects. Depending on your configuration, it can use different set of attributes, the default one being ObjectId for single-forest and objectSID/msExchMasterAccountSid for account/resource forest configuration. So check what you are using and then match the values between O365 mailboxes and the on-prem forest(s).