I recently completed a migration from Exchange 2013 to Exchange 2016 and after the Exchange 2013 server was uninstalled that’s when the issues with OAB started. I confirmed that all the mailbox databases were set with the Offline Address Book.

I confirmed that the OAB Virtual Directory was set with the correct URL.

I ran the Outlook Test Email AutoConfiguration and noticed that in the output it didn’t have a line for OAB.
It turns out that the Offline Address Book needed to be set for Global Web Distribution.
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Get-OfflineAddressBook | fl name, *webdist* |

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Get-OfflineAddressBook | Set-OfflineAddressBook -GlobalWebDistributionEnabled $True |
Now when I check the OAB both the Web Distribution settings have been enabled.

You will need to perform an IISreset for the settings to be applied.