The simplest approach would be to alias the "Days Open" column and apply the format mask to the alias
column days_open heading 'Days|Open' format a4;
select O_Num, OOpenDate, to_char(sysdate-OOpenDate, '999') days_open
from Orders
where Status = 'Open';
Otherwise, the column name in your column
command would need to exactly match the expression in your SELECT statement (including the to_char
, any spaces, etc.) That's possible but it generally makes things rather hard to maintain.