The administration and maintenance of Microsoft Exchange Server to ensure secure, reliable, and efficient email and collaboration services across an organization.
If you would like to disable OWA access for users in specified OUs, I suppose you may use the following command in Exchange Management Shell:
Get-mailbox -OrganizationalUnit "Contoso.com/Tailspintoys" | Set-CASMailbox -OWAEnabled:$false
It would disable OWA for all users in the Tailspintoys OU.
When users enter their credentials to log in OWA, they would see the following error:
You may also do it in Exchange Admin Center.
Here is a link on this topic for your reference: Enable or disable Outlook on the web access to mailboxes in Exchange Server
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