Try to do it like this:
Login to your Oleg firmware with ssh.
Switch to /tmp and download the openwrt trx file :
cd /tmp
wget http://downloads.openwrt.org/whiterussian/rc4/micro/openwrt-brcm-2.4-jffs2-4MB.trx
When the file is download issue the following command:
mtd -e linux -r write openwrt-brcm-2.4-jffs2-4MB.trx linux
Now the flash will be erased and rewritten with OpenWRT.
I have chosen the "micro" version of RC4, this is a version with some ppp/pppoe and so on stripped, stuff you wont need on your non-router device :-)