• Use the "Scheduled Flow" event trigger card to start your Workflow
• Use Okta Connected App card "List Users With Search" selecting the "Stream Matching Records" Result Set option
• Select an existing or create a new Child Flow
• In Child Flow set an input object named "user" (can really be any name you want)
• In Child Flow add Card "Object Get Multiple"
• Drag "user" object to input of "Object Get Multiple" card
• Enter "Record.Login" as Output of "Object Get Multiple" card and this will contain the "username" you are looking for.

That is it! Now you can do whatever it is you wanted to do with each user's username in the Child Flow.

Rather than using a schedule and looping all users or subset of users, consider using a different Okta event trigger card like "User Okta Profile Updated", "User Sign In Attempt", "User Activated" or something else to be more JIT (just in time). If you go that route the trigger card will return one object at a time and you can get the "username" by grabbing the "Alternate ID" from the "Okta User" output object and then you can do something right in that same Workflow or pass it to a Child Flow.