Hello Ali,

Yes, it’s possible to move your Odoo Online database to an on-premises setup, but there are a few important details to know:

1. Database Export from Odoo Online:
   • You can download your full database backup from your Odoo Online dashboard (Settings → Manage Databases → Download Backup).
   • This backup will include your custom modules and data.
2. Matching Version Required:
   • Make sure the version on your new on-premise server is exactly the same as your online version. Note that ONLY Odoo LTS (Long Term Supported) versions, that is Odoo 16.0, Odoo 17.0, Odoo 18.0 and Odoo 19.0 are supported versions for On Premise - this means you CANNOT migrate until your database is upgraded to Odoo 19.0
   • Otherwise, the database may not load properly.
3. Installing Odoo On-Premises:
   • Download the corresponding Odoo Community or Enterprise package from www.odoo.com/page/download.
   • If you’re using Enterprise, you’ll need valid subscription access to get the Enterprise source code.
4. Restoring the Backup:
   • Once Odoo is installed, open the web interface at http://<your-server-ip>:8069/web/database/manager.
   • Use the Restore Database option and upload your backup file.
5. Post-Restoration Checks:
   • Reinstall any custom modules you were using.
   • Verify that your addons paths in odoo.conf are correctly configured.
   • Check all external integrations (email servers, payment gateways, etc.) since credentials won’t migrate automatically.

Hi,

Please refer to the solved forum:

1. https://www.odoo.com/id_ID/forum/help-1/odoo-online-to-on-premise-hosting-241277

2. https://www.odoo.com/id_ID/forum/help-1/moving-to-on-premise-234379

Hope it helps.

You can download the source for on-premise hosting from https://www.odoo.com/page/download. 

Unless you're an Odoo partner I doubt you will have access to the GitHub-repository https://github.com/odoo/enterprise though. 
The reason why I mention this is that, as far as I remember, you can download latest x.0 versions (i.e. 16.0, 17.0 and 18.0 - soon 19.0; aka major releases) sources from the download pages only.

That being said, you probably want to await the release of 19.0 and the respective upgrade-option in Odoo Online for this version, so you can then import your (then 19.0) database into your local/on-premise setup.