All user sessions

SET LINESIZE 300
SET PAGESIZE 9999

COLUMN sid               FORMAT 99999      HEADING 'SID'
COLUMN serial_id         FORMAT 99999999   HEADING 'Serial ID'
COLUMN session_status    FORMAT a9         HEADING 'Status'          JUSTIFY right
COLUMN oracle_username   FORMAT a14        HEADING 'Oracle User'     JUSTIFY right
COLUMN os_username       FORMAT a12        HEADING 'O/S User'        JUSTIFY right
COLUMN os_pid            FORMAT 9999999    HEADING 'O/S PID'         JUSTIFY right
COLUMN session_program   FORMAT a26        HEADING 'Session Program' TRUNC
COLUMN session_terminal  FORMAT a10        HEADING 'Terminal'        JUSTIFY right
COLUMN session_machine   FORMAT a19        HEADING 'Machine'         JUSTIFY right

prompt 
prompt +----------------------------------------------------+
prompt | User Sessions (All)                                |
prompt +----------------------------------------------------+

SELECT
    s.sid                sid
  , s.serial#            serial_id
  , lpad(s.status,9)     session_status
  , lpad(s.username,14)  oracle_username
  , lpad(s.osuser,12)    os_username
  , lpad(p.spid,7)       os_pid
  , s.program            session_program
  , lpad(s.terminal,10)  session_terminal
  , lpad(s.machine,19)   session_machine
  , to_char(s.logon_time,'dd/mm/yyyy hh24:mi:ss')   slogon_time
FROM 
    v$process p
  , v$session s
WHERE p.addr (+) = s.paddr
    and s.type = 'USER'
ORDER BY sid
/

Sessions SID, PID and username

select p.pid, p.spid, s.username 
           from v$process p, v$session s 
           where p.addr = s.paddr
/