A common goal is to repurpose an old PC as an OMV server. This is straightforward but it is preferable not to have a powerful graphics card eating up power on the PCIe slot. Here is how I managed to get a motherboard (with no on-board GPU) to operate without a graphics card.
You will have to buy a cheap PCI card such as this one, but it can eventually be removed completely at the end of the process.
The core problem here is that you need graphics to install OMV but if you then remove the card and boot OMV (you will need to change your BIOS to "no halt on error" before booting without a GPU), you will have no services (no web admin, no SMB, no NFS, no SSH etc...). This is because without a graphics card on the PCIe slot, the motherboard ethernet adapter gets re-assigned because it also sits on the PCIe bus. Because of this hardware change, OMV will be essentially broken as even though it will be running, it will not recognise the ethernet adaptor due to the PCIe bus change.
This re-assigned ethernet adapter issue can be fixed easily with the command "omv-firstaid" but in order to run it successfully we need a terminal and a graphics card! This is the catch-22 situation at the heart of the problem.
To proceed, get hold of a PCI (not PCIe) VGA graphics card like this one. It is essential that it fits one of the smaller PCI slots and not the main PCIe slot usually associated with GPUs. This will give you basic VGA graphics and allow you to run "omv-firstaid" to re-connect OMV with the on-board ethernet adapter.
You can now either run your server with the PCI VGA card in place, or you can remove it and run without any graphics at all. If you remove it, you will have no local terminal but if the need arises just put the VGA PCI card back in.