Try this:
Basically just add the total of CNT_OPENED and CNT_CLOSED, then whichever you want to take the percentage of, multiply that by 100 and divide by the sum.
For instance, CNT_OPENED = 219 and CNT_CLOSED = 201 so the total is 420. Multiply CNT_OPENED by 100 and then divide by 420 -> (219 * 100) / 420 = 52. Do the same thing with CNT_CLOSED.
Note that this WILL result in an exception if both CNT_OPENED and CNT_CLOSED are 0.
SELECT 'Data'
||','||TO_CHAR(D.DTIME_DAY,'MM/dd/yyyy')
||','||NVL(o.CNT_OPENED,0) --as cnt_opened
||','||(NVL(o.CNT_OPENED,0) * 100) / (NVL(o.CNT_OPENED,0) + NVL(o.CNT_CLOSED,0)) || '%'
||','||NVL(c.CNT_CLOSED,0) --as cnt_closed
||','||(NVL(o.CNT_CLOSED,0) * 100) / (NVL(o.CNT_OPENED,0) + NVL(o.CNT_CLOSED,0)) || '%'
That will also potentially give you a million decimal places, so if you only want to take it out to a couple, simply use the TRUNC function and specify your precision (2 decimal places in this case):
TRUNC((NVL(o.CNT_OPENED,0) * 100) / (NVL(o.CNT_OPENED,0) + NVL(o.CNT_CLOSED,0)), 2)