Just Enough to be Dangerous

...probably best describes my knowledge of Oracle admin. However, these are some interesting tidbits I have found on the web that help you see what is going on in your database:

How long a transaction will take

You can estimate how long a rollback will take by polling 

SELECT V.USED_UBLK, q.sql_fulltext, S.* 
FROM V$TRANSACTION  V, v$session S, v$sql q 
where 
V.addr = S.taddr 
and q.sql_id = s.sql_id 

and seeing the value of USED_UBLK (from Ask Tom).

If this value is going up, that means the transaction is progressing (see here). if it's going down, this may indicate that a rollback is taking place. Estimating how long it takes to reach zero will give you a good idea how long you must wait.

Which are the long running transactions

long running transactions: 

select * 
from v$session_longops 
where target = 'MDS.FXVOL_CONTENT'
and  sofar != totalwork 

or whichever table you want to see. Taken from the Oracle website here.
(Text in bold added after a DBA friend recommended it to me).

Which transactions are blocked and which are blocking

See blocked processes with: 

select 
    sid, 
    sql_text 
from 
    v$session s, 
    v$sql q 
where 
    sid in 
    (select 
       sid 
    from 
       v$session 
   where 
       state in ('WAITING') 
   and 
       wait_class != 'Idle' 
   and 
       event='enq: TX - row lock contention' 
   and 
      (q.sql_id = s.sql_id or q.sql_id = s.prev_sql_id)); 

The blocking transaction is given with:

select 
    blocking_session, 
    sid,    serial#, 
    wait_class, 
    seconds_in_wait 
from 
    v$session 
where 
    blocking_session is not NULL 
order by 
    blocking_session; 

(SQL taken from here).

With this SID, you can see what SQL is running with this:

select 
    sid, 
    sql_text 
from 
    v$session s, 
    v$sql q 
where 
    q.sql_id = s.sql_id 
and s.sid=SID_YOU_JUST_FOUND 

All Active Queries

SELECT 
ss.program, 
SS.SERIAL#, 
W.SID, 
SUBSTR(SS.USERNAME,1,8) USERNAME, 
SS.OSUSER "USER", 
AR.MODULE || ' @ ' || SS.machine CLIENT, 
SS.PROCESS PID, 
TO_CHAR(AR.LAST_LOAD_TIME, 'DD-Mon HH24:MM:SS') LOAD_TIME, 
AR.DISK_READS DISK_READS, 
AR.BUFFER_GETS BUFFER_GETS, 
SUBSTR(SS.LOCKWAIT,1,10) LOCKWAIT, 
W.EVENT EVENT, 
SS.status, 
AR.SQL_fullTEXT SQL 
FROM V$SESSION_WAIT W, 
V$SQLAREA AR, 
V$SESSION SS, 
v$timer T 
WHERE SS.SQL_ADDRESS = AR.ADDRESS 
AND SS.SQL_HASH_VALUE = AR.HASH_VALUE 
AND SS.SID = w.SID (+) 
AND ss.STATUS = 'ACTIVE' 
AND W.EVENT != 'client message' 
ORDER BY  SS.LOCKWAIT ASC, SS.USERNAME, AR.DISK_READS DESC 

Script taken from here.