I personally do not intend to revert back to the stock OS, but if you want to do it, you can follow this guide here: Reinstall stock firmware on qhora-322 - Installing and Using OpenWrt - OpenWrt Forum. Looks pretty good to me. 
If you want to save yourself some hassle though (Edit: So you do not have to decrypt the stock firmware file), you can backup the two partitions to your USB flash drive, before executing the last two commands in section 5 of my guide. After mounting the USB flash drive, do this:
dd if=/dev/mmcblk0p1 of=/mnt/USB/mmcblk0p1_stock.img
dd if=/dev/mmcblk0p3 of=/mnt/USB/mmcblk0p3_stock.img
Once the backup is finished, proceed with writing OpenWrt:
dd if=0.img of=/dev/mmcblk0p1
dd if=1.img of=/dev/mmcblk0p3
Have a nice day!