sqlplus / as sysdba, ORA-01031: insufficient privileges

Follow following steps to correct problem:
1-) go to $ORACLE_HOME/rdbms/lib
2-) correct dba OS user group name in config.s file:
a) type "id" command in shell in order to get group name of your OS user
b)correct group name in config.s file. It is written just after the ".string"
.csect H.12.NO_SYMBOL{RO}, 3
.string "dbat"
# End csect H.12.NO_SYMBOL{RO}
.csect H.14.NO_SYMBOL{RO}, 3
.string "dbat"
# End csect H.14.NO_SYMBOL{RO}
c) run following commands to backup & compile:
mv config.o config.o.org
make -f ins_rdbms.mk ioracle

Note: sometimes dba OS dba group is correct in config.s file so you dont need to change anything and compile (next step) solves the problem.