sqlplus ORA-01031 Insufficeint privileges

I am trying to start sqlplus on Oracle 10gR2 (on Sun server). Keep getting error ORA-01031 Insufficient privileges. Here's how I am executing this from the start as oracle user:

login as: bill
Using keyboard-interactive authentication.
Password:
Last login: Tue Oct 24 13:47:48 2006 from 192.168.3.38
Welcome to the Sun Fire-V240 Server "Jacob"!
-bash-3.00$ su oracle
Password:
$ cd /export/ORACLE_HOME/bin
$ sqlplus /nolog

SQL*Plus: Release 10.2.0.1.0 - Production on Tue Oct 24 13:54:11 2006

Copyright (c) 1982, 2005, Oracle. All rights reserved.

SQL> connect sys as sysdba
Enter password:
ERROR:
ORA-01031: insufficient privileges

SQL> connect oracle/oracle as sysdba
ERROR:
ORA-01031: insufficient privileges

SQL>

I've tried several combinations of login attempts but keep getting the same error. I've also referenced others' posts here at this site but don't see any solution that I can extract . Any ideas from anyone who has experienced this problem, I'd appreciate some help.

use su - oracle for one

also make sure that oracle is in the dba group (or whichever group was specified during install)