How to check blocking and blocked sessions in Oracle

Show blocker → blocked pairs from V$SESSION. BLOCKING_SESSION is populated when a session is waiting on a lock held by another session; join it back to V$SESSION to see the whole picture.

SELECT b.sid     AS blocker_sid,   b.serial# AS blocker_serial,   b.username AS blocker_user,
       b.machine AS blocker_machine, b.program AS blocker_program,
       w.sid     AS blocked_sid,   w.serial# AS blocked_serial,   w.username AS blocked_user,
       w.event   AS blocked_wait_event, w.seconds_in_wait AS wait_sec,
       w.sql_id  AS blocked_sql_id
FROM   v$session w
JOIN   v$session b ON b.sid = w.blocking_session
WHERE  w.blocking_session IS NOT NULL
ORDER BY w.seconds_in_wait DESC;