issue:

bash-3.2# sqlplus xxxx/xxxx

SQL*Plus: Release 10.2.0.2.0 - Production on Wed Apr 2 02:30:05 2003

Copyright (c) 1982, 2005, Oracle. All Rights Reserved.

ERROR:
ORA-01034: ORACLE not available
ORA-27121: unable to determine size of shared memory segment
SVR4 Error: 13: Permission denied

This is caused by Oracle installer not setting setuid
on $ORACLE_HOME/bin/oracle.

Fix:
# cd $ORACLE_HOME/bin
# chmod 6751 oracle