How do you show SQL executing on an Oracle database?

Most of the info is available in v$session ... and the SQL Text can be grabbed from v$sql or v$sqltext_with_newlines...

Here is a query that I often use that formats In-flight SQL, longest running at top.

-- In Flight SQL 
SELECT nvl(ses.username,'ORACLE PROC')||' ('||ses.sid||')' USERNAME,
       SID,   
       MACHINE, 
       REPLACE(SQL.SQL_TEXT,CHR(10),'') STMT, 
      ltrim(to_char(floor(SES.LAST_CALL_ET/3600), '09')) || ':'
       || ltrim(to_char(floor(mod(SES.LAST_CALL_ET, 3600)/60), '09')) || ':'
       || ltrim(to_char(mod(SES.LAST_CALL_ET, 60), '09'))    RUNT 
  FROM V$SESSION SES,   
       V$SQLtext_with_newlines SQL 
 where SES.STATUS = 'ACTIVE'
   and SES.USERNAME is not null
   and SES.SQL_ADDRESS    = SQL.ADDRESS 
   and SES.SQL_HASH_VALUE = SQL.HASH_VALUE 
   and Ses.AUDSID <> userenv('SESSIONID') 
 order by runt desc, 1,sql.piece;

Good answer found here.

Execute the following sql:

select x.sid
      ,x.serial#
      ,x.username
      ,x.sql_id
      ,x.sql_child_number
      ,optimizer_mode
      ,hash_value
      ,address
      ,sql_text
from   v$sqlarea sqlarea
      ,v$session x
where  x.sql_hash_value = sqlarea.hash_value
and    x.sql_address    = sqlarea.address
and    x.username       is not null;

If the output is unreadable, change the LINESIZE (take from here):

SET LINESIZE 20000

If the sql above does not work, you might need to log in as sysdba:

sqlplus '/as sysdba'