To define the email address to which the overdue invoice reminder is being sent in Odoo Online, you can follow these steps:

1. Login to your Odoo Online account and go to the "Invoicing" app.
2. Click on the "Customers" tab and select the customer for which you want to set up the reminder email.
3. In the customer's profile, click on the "More" dropdown menu and select "Send Reminder".
4. In the "Send Reminder" window, you can set the email template, add any additional notes, and select the email address where you want the reminder to be sent. The email address will automatically be populated with the customer's email address, but you can change it if needed.
5. Once you have entered all the necessary information, click on the "Send" button to send the reminder email.

If you want to set up automatic reminders for overdue invoices, you can do so by going to the "Settings" menu in the Invoicing app, selecting "Configuration" and then "Payment Follow-up". From there, you can set up the frequency and content of the automatic reminders.