In diesem Fall ist die abgeleitete Tabelle, die einen Alias erfordert, diejenige, aus der Sie * auswählen.
Einrückungen helfen, dies klarer zu machen.
SELECT * FROM
(
(SELECT '1' AS `table`, `vid_req_timestamp` AS `timestamp`, `title` FROM `movies` WHERE `vid_req` = '1')
UNION
(SELECT '2' AS `table`, `ost_req_timestamp` AS `timestamp`, `title` FROM `movies` WHERE `ost_req` = '1')
) AS `some_table_name_lol_this_is_an_alias`
ORDER BY `timestamp` DESC