В общем пока так решил проблему:
[code]
if ($ssl_client_i_dn ~ (\\/O=([0-9]*)))
{
set $ssl_client_i_dn $2;
}
fastcgi_param SSL_CLIENT_I_DN_O $ssl_client_i_dn;
[/code]
Posted at Nginx Forum: http://forum.nginx.org/read.php?21,172482,172613#msg-172613