Oracle SQL for left outer join to rownum = 1 of another query?
Basically, I think you should just get the max date, if that is all you are looking for here, using your same filter, etc. You already have STATUS (=COMP) and WONUM (JOIN). If you needed the whole record from this table, and it was more complicated than this, I would recommend the oracle inline analytic functions with over/partition by logic to filter by the max date. 

SELECT *
FROM   WORKORDER
       LEFT OUTER JOIN (SELECT WONUM AS STATUSWONUM 
                               , STATUS AS STATUS
                               , MAX(CHANGEDATE) AS STATUSCHANGEDATE
                        FROM   WOSTATUSHISTORY
                        WHERE  STATUS = 'COMP'
                        GROUP  BY WONUM, STATUS)LASTCOMPLETE
         ON ( WORKORDER.WONUM = LASTCOMPLETE.STATUSWONUM ) 

;