Hey Michelle,
To do this, you'll need to set up a custom field in your CRM that is populated with the Opportunity Owner's email. Since contacts can have multiple Oppty's you would probably need to have this populate based on the date of the Most Recent Oppty. This would then sync down to Marketo on the contact, and you can use this field as a token in the "To Other Emails" field of the Send Alert flow step.