Ok. After couple of hours searching and trying everything possible, i figure it out. Here is little tutorial, how to do it.
1. Go to Archive.org
2. Next go to the GameSpy site and search couple of Demo disks. On some of them, there are really rare demos, like Fifa 06 RTFWC, Fifa WC 2006, Pes 2006 Demo, Nba 2k6 etc You just have to find the right OXM disk with the Demo you want. (for example : "OXM Demo Disc 61, September 2006" with the Playable demo of Saints Row.
3. On the Archive.org site, search in "software category" for the Demo disk, that you choose. (There are 2 types. First is classic OXM Demo disc and second is OXM Demo Disk Issue) I will get to the difference between them later.
4. Download it. btw. I would recommend download them via torrent. Some of them have around 7GB and torrents are faster + saver then classic direct download for me.
5. After everything is ready, use "wx360 1.6" to extract ISO file you just downloaded. You find the tutorial on YouTube how to do that.
6. So, finally you have everything fully extracted. And there goes the difference between OXM Demo Disc and OXM issue because if you want to play classic OXM Demo DIsc on X360, You must have JTAG or RTG (modded) console. If not, it will not work for you and there is no possibility to play the Demos. (btw. Xenia runs the classic OXM Demo discs, but the Demos itself not.) BUT if you have the OXM ISSUE Demo Disk, there is a way to separate the Demos itself from the disk, burn them to DVD or CD, and play them normally on your console. The Way to do this is (after you have done everything i previously mention here) : Go to your extracted OXM Demo folder, Content, "00000000000" folder, next go to "494D87F9" folder (you will have probably different numbers), next click the "00080000" folder and there you have all the playable Demo game files from the disk. Like i said, you can burn them to your CD or DVD using IMGBurn, and play them normally on your Xbox. If you dont know how to do this, just go to Digiex, and search for "How to burn images to disk", you will find it for sure. It works perfectly.
I hope that i help you a little bit with all this Mystery and Rare Demo mess. For sure not every demo must be in this OXM Disks, but if you cannot find some demo, there is a BIG change it will be on some of these OXMs. And the last thing, sorry for my English
