Can't drop UNDO TABLESPACE
I tried the above suggestions, but did not work, such as select SQL did not get results and I still cannot drop the undo table space.  The fundamental issue is that the old undo table space is still registered in Oracle as its undo, therefore we need to change it to the new undo table space.

Here are what I did and it worked:


create a new UNDO table space:


CREATE SMALLFILE UNDO TABLESPACE "UNDO" 
DATAFILE '+DATA/t2/datafile/undo_01.dbf' SIZE 10G 
REUSE AUTOEXTEND ON NEXT 500M MAXSIZE 10G;



Switch the new UNDO to the "official" undo table space:


alter system set undo_tablespace='UNDO' scope=both;



drop the old undo table space:


DROP TABLESPACE UNDOTBS1 INCLUDING CONTENTS AND DATAFILES;


hope this helps.