Peculiar Oracle outer join syntax case
I tried to perform the mechanical process. I hope I remember it right.

This leads to:

SELECT ...
FROM A
         join B on A.A_ID = B.A_ID
    left join C on B.B_ID = C.A_ID and B.B_KEY = C.B_KEY and 'CONSTANT' = C.X_ID
    left join D on C.C_ID = D.C_ID
    left join E on B.A_ID = E.A_ID and B.B_KEY = E.B_KEY


In short I think Leigh Riffel's answer is correct.

Note

in old days the rule to memorize was:
oracle where A.a = B.b (+)
becomes A.a *= B.b in SQL-Server old syntax the plus goes to the opposite side and becomes a star,
which means A left join B on A.a = B.b