How can a deal's owner receive a notification via an automation?

You can set up a notification step to alert the deal owner that a contact/deal has passed a certain point. You can use the personalization tag %DEAL_OWNER_EMAIL% in the 'send to' field of the notification step.

This will send a notification to the owner of that deal. If multiple deals exist for the contact, the most recently created deal's owner will be notified.