To set default value for a field in Odoo interface you have to activate the ‘developer mode’.

1. Click on your name on the upper right corner
2. Click on ‘About OpenERP’
3. Click on ‘Activate the developer mode’

After that you should see a selection box on the top of each form.

Enter default value in a field and choose ‘Set Defaults’ from developer selection box.

You can also choose if it is default value to all users or only to you.