Nicks Solution Tweaked

WITH set_sizes AS (
  SELECT x, COUNT(*) AS set_size  -- "size" is a reserved keyword in Oracle
  FROM t
  GROUP BY x
)
, intersection_sizes AS (
  SELECT
      sub.x     sub_x
    , super.x   super_x
    , COUNT(*)  intersection_size
  FROM
                t sub
    INNER JOIN  t super
      ON  sub.y = super.y
      AND sub.x < super.x
  GROUP BY
      sub.x
    , super.x
)
SELECT ss.x AS sub_x, decode(ss.x,xs.sub_x,xs.super_x,xs.sub_x) AS super_x
FROM
              set_sizes           ss
  INNER JOIN  intersection_sizes  xs
    ON  (ss.x = xs.sub_x OR ss.x = xs.super_x)
    AND ss.set_size = xs.intersection_size
;