OMV supports running from a USB stick. Not ideal in all situations, but I ran it successfully that way for many years. Another option for install is to install a base Debian system and then install OMV on top of it. Both are supported install methods. I believe you can create your separate partitions as part of the deb install and then OMV only installs on the deb partition.
Please confirm all of this info from the docs as this is my recollection and I have not validated it recently.