Oracle custom week of the year numbering based on first Monday of the year
ISO Definition is here, You need to check if Jan-01 is Monday, if it is, then you can use ISO week format model 'IW'. If it isn't you can still use that but subtract 1 from that value. 

WITH x AS (SELECT to_char(trunc(SYSDATE,'YYYY'),'DY') y_b FROM dual)
,y AS (SELECT to_date('01-05-15','mm-dd-rr') AS d FROM dual UNION ALL
       SELECT to_date('01-12-15','mm-dd-rr') AS d FROM dual UNION ALL 
       SELECT to_date('01-19-15','mm-dd-rr') AS d FROM dual) 
SELECT y.d your_date
      ,CASE WHEN x.y_b = 'MON' 
            THEN to_number(to_char(y.d,'IW')) 
            ELSE   to_number(to_char(y.d,'IW'))-1 
        END week_num
FROM x,y;


I tested briefly and it looks ok to me, but you should test as well.