Which version oscam for OpenPli 2.1 dreambox 800hd

Check out the latest version of Oscam 11572, for mipsel architecture.

Mipsel is also in your old DM800 HD -> https://en.wikipedia.org/wiki/Dreambox#Table

However, I don't know if it will run on the old OpenPLi 2.1 and the old Linux kernel.

First try the IPK package for the new OpenPLi 7.2, for mipsel architecture. If it doesn't help, just try download a individual binary file (for mipsel architecture), upload it to the /usr/bin folder, then assign the attributes chmod a+x /usr/bin/oscam and then try to run the binary as /usr/bin/oscam --help. If the oscam binary file works... also add there some "init.d" startup script. Procedure for init.d script can be found here: Tutorial : How to install softcam (OSCam/CCCam) manually, if you hate ipk packages