DBA_WAITERS shows all the sessions that are waiting for a lock. In an Oracle RAC environment, this only applies if the waiter is on the same instance.

Column Datatype NULL Description
WAITING_SESSION NUMBER   The waiting session
HOLDING_SESSION NUMBER   The holding session
LOCK_TYPE VARCHAR2(26)   The lock type
MODE_HELD VARCHAR2(40)   The mode held
MODE_REQUESTED VARCHAR2(40)   The mode requested
LOCK_ID1 VARCHAR2(40)   Lock ID 1
LOCK_ID2 VARCHAR2(40)   Lock ID 2