I am getting the following ora error message when using sqlplus to login to oracle.   I am logged in as my own unix account and not oracle.

gguser@prodhost:/u01/home/gguser > sqlplus ggadmin

SQL*Plus: Release 9.2.0.4.0 - Production on Wed Apr 18 13:23:00 2009

Copyright (c) 1982, 2002, Oracle Corporation. All rights reserved.

Enter password:
ERROR:
ORA-01034: ORACLE not available
ORA-27123: unable to attach to shared memory segment
Error: 13: Permission denied

Here is the solution to this ora error message:

#  cd $ORACLE_HOME/bin

#  ls -ltr oracle

-rwxr-xrwx 1 oracle dba 74053740 Sep 04 2003 oracle

The file permissions for "oracle" executable should be "-rwsr-s--x",  so that others can access the oracle environment from the same host.

You need to shutdown the oracle database and then issue the command:

cd $ORACLE_HOME/bin

chmod 6751 oracle
ls -lr oracle

rwsr-s--x 1 oracle dba 74053740 Sep 04 2003 oracle

After you do this, startup the oracle database and then try to connect .   This should have resolved the issue.

Hope this helps.