This is how i installed OMV on my thecus n5200xxx nas, since i am a noob concerning linux etc. don't make me responsible for anything.
Just wanted to share my way how i did it to get some more information out to the interwebs 
Most of the information i got is from this forum and google, so thanks for all the guides/how to's
Since the thecus mainboard does not have an vga output connector you have to solder one on the board yourself.
I used an old VGA connector socket i had on a vga-card from an old computer.
with this Pinout: http://all-pinout.blogspot.co.at/2011/10/vga-pin-out.html
Looks like this: 
You could also use a socket that can be soldered onto the board directly.
If you use something like the one i have, you need to check the pins since they arent 1-1, 2-2, 3-3 etc
In example first line (red marked) on this cable i have is gnd!
i replaced the thecus DOMs with a 2GB Transcend 44pin IDE DOM and upgraded the RAM with some 4GB Transcend DDR3 Sodimm, both from amazon.
on the thecus homepage you can find a compability list for RAM. Since the thecus OS is 32bit afaik theres only up to 2GB modules compatible according to that list.
i put the model number in google search and looked for 4gb modules of that model since the hardware supports x64 architecture. its an Atom D525 btw
Put a bootable installion-img on an usb-stick and half an hour later you have OMV on your thecus nas. guide for installation from usb-stick
After installation i had problems booting.
After grub started omv i only had [x.xxxx] not responding on my screen and after 3 times "not responding" it rebooted.
I do not know but somehow i managed to boot it (pressing enter so timeout in grub wouldn't run down worked sometime)
Some googling and i found this: http://forums.openmediavault.org/viewtopic.php?f=12&t=188
so i added ata_piix ide_generic and pata_legacy to /etc/initramfs-tools/modules and updated the system with "update-initramfs -u" like you can read in that thread.
in the web-ui i installed omv-extras and the backports kernel
Since then i dont have trouble booting from the DOM, as said im a noob so please dont ask why 
[had to reinstall OMV today because i fu**ed up my webgui and with this changes it solved the "not responding" on boot problem again]
Harddisk configuration is like i had in my thecus system. 4xdisks in a raid5 and a small disk for system-stuff.
OMV imported the Raid5 without any problems, i had a backup of the important stuff which you should obviously do in case it doesn't work