If I'm UNIONing 2 queries can I use a dynamic ORDER BY like this?

order by decode(sortby,'1',substr(last_name||', '||first_name||' '||mi,1,24),
                              '2',styp_code,
                              '3',fund_code);

**2 columns on the 2nd query are different than the 1st. and it just so happens to be what I'm trying to sort by.

I get this error "order by item must be the number SELECT-list expression".
And tried:
Cause: A column alias was used in the ORDER BY clause of a SELECT statement that uses a UNION, INTERSECT, or MINUS set operator. This is not allowed. In such cases, expressions in the ORDER BY clause must be unsigned integers that designate the ordinal positions of select-list items.

Action: Change the alias in the ORDER BY clause to an unsigned integer that designates the ordinal position of the select item in question.

I tried:
order by decode(sortby,'1',3,
                              '2',4,
                              '3',8);
but it gives the same error message.

Also, without the UNION and the 2nd query the ORDER BY works fine. And if I remove the decode while still using the UNION command and just do ORDER BY 3,4,8 it works fine too.