How to export a mailbox from a tenant enabled with OME encryption

You have to export the mailbox from OME encryption enabled tenant to a .pst file and then use Powershell to decrypt via Unprotect-RMSFile

Please refer to PowerShell articles below,
Unprotect-RMSFile (AzureInformationProtection) | Microsoft Docs
Use PowerShell with the Azure Information Protection unified labeling client | Microsoft Docs

Note: You can also refer to the articles below for mailbox migration 
How to migrate mailboxes from one Microsoft 365 or Office 365 organization to another | Microsoft Docs
Cross-tenant mailbox migration - Microsoft 365 Enterprise | Microsoft Docs