Thank you for posting your inquiry to Okta Support Community Portal.
Currently, there's no out of the box report to check who changed their AD password via Okta, but you can accomplish this via the System log by using this search filter "eventType eq "user.account.update_password" " and then downloading the CSV file.
Also, you could try using the Active Directory Audit logs for password changes against the Okta Service Account.
If you would like to see such functionality in Okta, the best route to pursue this is via a feature request. The best way to file feature requests would be from the Community Idea site.
I would disagree with "eventType eq "user.account.update_password" " as there are basically no results. I found that if you use this: eventType eq "system.agent.ad.reset_user_password" and outcome.result eq "SUCCESS" you get what appear to be good results. Note that I am finding duplicate entries at times, but this should suffice.