Most important first fact: only Raspberry Pi 2 and newer are supported (ARMv7), no RPi 1 and RPi 1 based devices.
OMV is not very well supported as there is just no OMV dev active here in Plex community. We recommend a plain Raspbian installation, the default Debian distribution for the Raspberry Pis.
You need clients that do not require transcoding at all.
All this and most other stuff is discussed in our setup guide:
