SQL & PL/SQL

I have a user parameter where the user is supposed to choose what column they want to sort by.
And they can choose between k.personnr and k.personnavn. I am trying to do this with a decode function.
Now this would work fine if there was no union between these select statements.
But with the union I get a ORA-01785, where they tell me to put in a number.

Here is the sql:

SELECT
k.personnr as PERSON ,
k.personnavn as NAVN,
r.belop as SUM,
concat(to_char(r.bilagdato,'MM'), to_char(r.bilagdato,'YYYY')) AS MND,
PERIODE(concat(to_char(r.bilagdato,'MM'), to_char(r.bilagdato,'YYYY'))) AS PERIODE
FROM
reskontro r,
klient k,
kontoplan p,
konto c
WHERE
r.distriktnr='21'
and r.distriktnr=c.kontoid
and r.distriktnr=k.distriktnr
and r.personnr=k.personnr
and r.kontonr=p.kontonr
and (((r.bilagdato between :p_fra_dato and :p_til_dato) and (:p_fra_dato <= c.overfort) and (:p_til_dato <= c.overfort))
or ((r.bilagdato between :p_fra_dato and c.overfort) and (:p_fra_dato <= c.overfort) and (:p_til_dato >= c.overfort)))
UNION ALL
SELECT
k.personnr as PERSON ,
k.personnavn as NAVN,
(t.belop/100) as SUM,
concat(to_char(t.bilagdato,'MM'), to_char(t.bilagdato,'YYYY')) AS MND,
PERIODE(concat(to_char(t.bilagdato,'MM'), to_char(t.bilagdato,'YYYY')),:P_HORISONTAL) AS PERIODE
FROM
transaksjon t,
klient k,
kontoplan p
WHERE
t.distriktnr='21'
and t.distriktnr=k.distriktnr
and t.personnr=k.personnr
and t.kontonr=p.kontonr
and t.bilagdato between :p_fra_dato and :p_til_dato

This is the order by clause I would like to use:
order by decode(:p_order_by,'Personnavn',k.personnavn,k.personnr)

I also get an error when I use:
order by decode(:p_order_by,'Personnavn',2,1)

Can anybody tell me how to work around this problem?