WITH results AS (
    SELECT
        or.original_timestamp,
        or.original_result,
        ov.override_timestamp,
        ov.override_result,
        ov.override_reason,
        lag(or.original_result)
          OVER (order by or.original_timestamp DESC) AS previous_result
    FROM
        original or
        LEFT JOIN override ov ON ov.override_original_id = original.id
    WHERE
        or.original_foo_id = 1
        AND or.original_meta_data_id = 400
)
SELECT
    original_timestamp,
    original_result,
    override_timestamp,
    override_result,
    override_reason,
    previous_result
FROM results
WHERE original_result != previous_result
ORDER BY original_timestamp DESC;

original table

id | original_foo_id | original_timestamp | original_result | meta_data_id
---+--------+-----------+-------------------------------------------------
1  | 1               | 2016-01-01          | 0               | 400
3  | 1               | 2016-01-02          | 0               | 400
4  | 1               | 2016-01-02          | 0               | 400
5  | 1               | 2016-01-03          | 1               | 400

override table

id | override_original_id | override_timestamp | override_result | override_reason
---+-------------+-----------+--------+-------------------------------------------
1  | 1                    | 2016-01-04         | 2               | Tests failed.
2  | 2                    | 2016-01-02         | 1               | Some reason.

change_timestamp | change_result | change_reason
--------------------------------------------------
2016-01-04       | 2             | Tests failed.
2016-01-03       | 1             | Auto
2016-01-01       | 0             | Auto