SQL> startup
ORACLE instance started.
Total System Global Area 3170893824 bytes
Fixed Size 2043968 bytes
Variable Size 2030047168 bytes
Database Buffers 1124073472 bytes
Redo Buffers 14729216 bytes
Database mounted.
ORA-01092: ORACLE instance terminated. Disconnection forced
ORA-30012: undo tablespace 'UNDOTBS01' does not exist or of wrong type
Process ID: 19872
Session ID: 1218 Serial number: 3
Solution
[oracle@eux3300392 dbs]$ vi initps82live.ora
undo_management=MANUAL
SQL> startup
Total System Global Area 6313463808 bytes
Fixed Size 2264976 bytes
Variable Size 1677721712 bytes
Database Buffers 4613734400 bytes
Redo Buffers 19742720 bytes
Database mounted.
Database opened.
SQL>
4. create a new undo tablespace, e.g.:
CREATE UNDO TABLESPACE "UNDOTBS3" DATAFILE '+DATA' SIZE 10880M AUTOEXTEND ON NEXT 5120K MAXSIZE 32767M
5. shutdown the database
6. change UNDO_MANAGEMENT=AUTO in your init file and set UNDO_TABLESPACE=UNDOTBS (or whatever name you specified during the undo tablespace creation)
7. connect "/ as sysdba" and startup the database using this init file (as in step 2).
Now the database will open using the new undo tablespace and it would have created new undo segments in this tablespace.