Oracle Database case study-failed to log on to Oracle database-oracle user not added to DBA User Group

1.1 symptom Description: failed to log on to the database. The system displays the following error message:
% Sqlplus/as sysdba

SQL*Plus: Release 10.2.0.4.0 - Production on Thu Aug 6 10:39:23 2009

Copyright (c) 1982, 2007, Oracle.  All Rights Reserved.

ERROR:

ORA-01031: insufficient privileges

1.2 possible causes OracleThe user is not added DBAUser Group.
Note:
In the security reinforcement procedure, modify OracleIf the user group is not added DBAGroup OracleUser's DBAThe Group is lost. 1.3 check positioning ideas OracleUser Group information.
% Id oracle

uid=1009(oracle) gid=998(oinstall) groups=998(oinstall),1000(omcsysm)

Shown above OracleThe user is not added DBAUser Group. 1.4 procedure RootThe machine on which the user logs on to the database. Set OracleAdd User DBAUser Group.

# Usermod-g oinstall-G dba, omcsysm-I oracle

Log out.

# Exit

To OracleThe machine on which the user logs on to the database. To SysdbaThe user connects to the database.

% Sqlplus/as sysdba

The logon is successful. The system displays the following message:

SQL*Plus: Release 10.2.0.4.0 - Production on Thu Aug 6 16:32:06 2009 

Copyright (c) 1982, 2007, Oracle.  All Rights Reserved. 

Connected to an idle instance.