"You should almost always index foreign keys.
The only exception is when the matching unique or primary key is never updated or deleted."When a foreign key is unindexed, DML on the parent primary key results in a share row exclusive table lock
(or share-subexclusive table lock, SSX) on the child table, preventing DML from other transactions
against the child table. If the DML affects several rows in the parent table, the lock on the child table
is obtained and released immediately for each row in turn. Despite the speed of the lock-release process,
this can cause significant amounts of contention on the child table during periods of
heavy update/delete activity on the parent table.When a foreign key is indexed, DML on the parent primary key results in a row share table lock
(or subshare table lock, SS) on the child table. This type of lock prevents other transactions
from issuing whole table locks on the child table, but does not block DML on either the parent or
the child table. Only the rows relating to the parent primary key are locked in the child table.

Script:

REM  List foreign keys with no matching index on child table - causes locksset linesize 150;col owner for a20;
col COLUMN_NAME for a20;SELECT c.owner,c.constraint_name,c.table_name,cc.column_name,c.statusFROM dba_constraints c, dba_cons_columns ccWHERE c.constraint_type = 'R'AND c.owner NOT IN('SYS','SYSTEM','SYSMAN','EXFSYS','WMSYS','OLAPSYS','OUTLN','DBSNMP','ORDSYS','ORDPLUGINS','MDSYS','CTXSYS','AURORA$ORB$UNAUTHENTICATED','XDB','FLOWS_030000','FLOWS_FILES')AND c.owner = cc.ownerAND c.constraint_name = cc.constraint_nameAND NOT EXISTS(SELECT 'x'FROM dba_ind_columns icWHERE     cc.owner = ic.table_ownerAND cc.table_name = ic.table_nameAND cc.column_name = ic.column_nameAND cc.position = ic.column_positionAND NOT EXISTS(SELECT owner, index_nameFROM dba_indexes iWHERE     i.table_owner = c.ownerAND i.index_Name = ic.index_nameAND i.owner = ic.index_ownerAND (i.status = 'UNUSABLE'OR i.partitioned = 'YES'AND EXISTS(SELECT 'x'FROM dba_ind_partitions ipWHERE status ='UNUSABLE'AND ip.index_owner =i.ownerAND ip.index_Name =i.index_nameUNION ALLSELECT 'x'FROM dba_ind_subpartitions ispWHERE status ='UNUSABLE'AND isp.index_owner =i.ownerAND isp.index_Name =i.index_name))))
ORDER BY 1, 2
/