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Bryan GordonBryan Gordon 

Outlook client + existing office 365 users prompted to sign in

We rolled out Office 365 in Okta.

The problem is that our users (all of whom were in Office 365 In-Cloud only users) are now prompted for a signon when they launch Outlook full client (some are 2010, 2013 and 2016 clients). The box is a small dialog box with email address and password.  When they enter their office 365 password, the dialog box returns asking for a password for Office 365.  They are unable to launch outlook after the Office 365 rollout in Okta.

What password are they supposed to be using? We've tried signing on with our Windows password and current office 365 password but the box just prompts the users anyway.
Kevin TurnerKevin Turner (Okta, Inc.)

It sounds like you've switch on Okta to perform the SSO to Office 365. I've also seen you're earlier questions about some users who are not in AD and that are O365 Cloud based users. If you have configured you're office 365 tenant to federate to Okta then all users with the username@company.com suffix will get redirected to login to Okta.

Those users who are not in AD should be created in the Okta tenant as "Okta mastered users" ie have a similar named user in Okta with a password in Okta that they authenticate with to Okta, that then provides them with SSO via federation to the Office 365 tenant.

So far as the other users who are using the rich clients they too are now trying to authenticate to Office 365 via the Okta federation by the sounds of things.

I suggest you contact support to help assist with the requirements.

James GarvinJames Garvin (Okta)
To dovetail onto what Kevin said, it could also be a caching issue with the Outlook client.  This KB from MS explains the Outlook caching and how to clear it

Duc CaoDuc Cao
We are having the same issue but difference is that we have a hybrid Exchange and O365 set up. The initial setup of the rich client, users get the prompt loop. If they keep on clicking cancel, it will eventually finish setting up and the prompt goes away. Any insight?
Joseph HalpyJoseph Halpy
We have experienced this issue as well along with another variant. 

1) The OKTA login prompt will loop over and over again.  It will not allow the user to enter their credentials.
2) The OKTA login prompt will pop then disappear.  The user is unable to enter their credentials.
Joel LeeJoel Lee

Did you try enabling Modern Authentication for Outlook?