Microsoft discontinued the "Microsoft Virtual Machine Converter 3.0" and no longer provides a download. The following approach is therefore essentially deprecated.
That said, experience shows you occasionally still need older methods — so the article stays.
1. Extract the .ova file with 7-Zip
2. If it contains a .vmdk, convert it with PowerShell using the "Microsoft Virtual Machine Converter 3.0"
2.1 Install the converter:
https://www.microsoft.com/en-us/download/details.aspx?id=42497
Then via PowerShell:
Import-Module 'C:\Program Files\Microsoft Virtual Machine Converter\MvmcCmdlet.psd1'
convertTo-MvmcVhd -SourceLiteralPath C:\Migration\sv-db.vmdk -DestinationLiteralPath d:\Hyper-V\VHDs -VhdType FixedHardDisk -VhdFormat vhdxUse -VhdType DynamicHardDisk to create a dynamically expanding disk — uses less space initially.
PowerShell doesn't natively support VMDK conversion. Install the Virtual Machine Converter first, then import the PowerShell module:
https://www.microsoft.com/en-us/download/confirmation.aspx?id=42497
Import-Module "C:\Program Files\Microsoft Virtual Machine Converter\MvmcCmdlet.psd1"
ConvertTo-MvmcVirtualHardDisk -SourceLiteralPath E:\myvm\source.vmdk -VhdType DynamicHardDisk -VhdFormat vhdx -destination E:\myvmH@ppy H@cking