This is an old post I wrote, but I'm rewriting it because I just had a customer that needed this tool.
I've created a small oerr "tool" for use in windows. I don't have databases on windows machines, but i do use windows as a client and when I get an error in sqlplus i don't wish to ssh to a linux/unix server and run oerr there.
The "tool" is a batch file with gawk and the message files from a 10g oracle installation. The zip file contain information about how to use other oracle version messages.
You can get the file from here.