Events
ID   Type    Date
1    On      01-MAY-12
1    Off     01-JUN-12
1    On      05-JUN-12

Events
ID   Type    Date
1    On      01-MAY-12
1    On      01-MAY-12
1    On      01-JUN-12
1    Off     01-JUL-12
1    Off     01-AUG-12
1    On      05-AUG-12

Events
ID   Type    Date
1    On      01-MAY-12
1    Off     01-JUL-12
1    On      01-AUG-12

SELECT id,
       type,
       "date"
  FROM ( SELECT id,
                LAG(type) OVER (PARTITION BY id ORDER BY "date") AS prev_type,
                type,
                "date"
           FROM events
       )
 WHERE type <> NVL(prev_type, '-')
 ORDER
    BY "date"
;