Wow, you're courageous to try running a PHP framework within Rails!
I don't know if this will help, but you might like to look at
RAILS_ROOT/public/.htaccess. That's where Rails rewrites all requests
to be handled by dispatch.cgi (or dispatch.fcgi depending on your
configuration). Check the Apache documentation: there may be a way of
configuring those RewriteRule's so that the directory where OSCommerce
lives isn't routed to Rails.