Yes, you can achieve this in Odoo using the Marketing Automation application. Here’s a general idea of how you can set this up:

1. Create a Target Audience: You can create a target audience based on specific criteria. In your case, you would want to target contacts who have made their first purchase through your website.
2. Set Up a Marketing Campaign: Once you have your target audience, you can set up a marketing campaign. This campaign can be configured to send an email with a discount to the targeted contacts.
3. Use Automated Actions: You can use Odoo’s automated actions to trigger the marketing campaign when a contact makes their first purchase. This could be set up as an automation rule that is triggered when a sales order is confirmed.