Something like this should get you started. I did this in the HR sample schema
SQL> select parent, child, level from (
2 select pt.table_name parent, ct.table_name child
3 from user_tables pt,
4 user_constraints pc,
5 user_constraints cc,
6 user_tables ct
7 where pt.table_name = pc.table_name
8 and pc.constraint_type IN( 'P', 'U' )
9 and cc.r_constraint_name = pc.constraint_name
10 and cc.constraint_type = 'R'
11 and ct.table_name = cc.table_name
12 and ct.table_name != pt.table_name
13 )
14 start with parent = 'COUNTRIES'
15 connect by nocycle prior child = parent;
PARENT CHILD LEVEL
------------------------------ ------------------------------ ----------
COUNTRIES LOCATIONS 1
LOCATIONS DEPARTMENTS 2
DEPARTMENTS EMPLOYEES 3
EMPLOYEES JOB_HISTORY 4
DEPARTMENTS JOB_HISTORY 3