1. If you connected the monitor to the Raspberry through HDMI, then you should have seen that there is still have a graphical interface.

2. But it’s really impossible to work with him, because
2.1. the image of raspberry is intentionally made so that no one uses it as a desktop. 
The reason lies in the fact that when working with Odoo, data must be written to the hard drive, and there is no HD for raspberry. There is only an SD card. And the SD card has a problem: each SD memory card cell has a limited number of write cycles, after which this memory cell fails and along with it pulls the entire SD card to the bottom (yes, because of one memory cell !).
Make raspberry to work as a server of equipment that does not store anything. It only accepts requests from connected clients, executes them, and returns the result. Therefore, recording to the SD card is not performed.
2.2. The second reason for blocking the interface is that users can independently connect the monitor and try to access the control of the Raspberry.

In general, connecting a monitor to Rasberry to work on it as a full-fledged computer is a bad idea.
I advise you to either put Odoo server hardware on a computer(you can use docker pos image, read on https://odoo-development.readthedocs.io about this), 

or buy an industrial computer with a built-in screen with a touchscreen and put odoo pos-server on it.
For example, like this: https://aliexpress.ru/item/32922499094.html