Is dropping foreign key constriants an event in Oracle?
The following trigger captures the drop constraint operation.

create or replace trigger foreign_drop after create or alter or drop on database
begin
  dbms_output.put_line('DDL Fired!');
end foreign_drop;
/
SQL&gt; set serveroutput on
SQL&gt; CREATE TABLE t2
( id number,
  CONSTRAINT fk_id
    FOREIGN KEY (id)
    REFERENCES t1(id)
);  2    3    4    5    6  
DDL Fired!

Table created.

SQL&gt; alter table t2 drop constraint fk_id;
DDL Fired!

Table altered.