Order by case insensitive in oracle

First of all, you can order by the UPPER (or LOWER) case of the column, but once you've done that, you then need to sort by the text itself to get the order on the initial letter; eg:

with sample_data as (select 'A' txt from dual union all
                     select 'B' txt from dual union all
                     select 'Y' txt from dual union all
                     select 'Z' txt from dual union all
                     select 'a' txt from dual union all
                     select 'b' txt from dual union all
                     select 'y' txt from dual union all
                     select 'z' txt from dual)
select txt
from   sample_data
order by upper(txt) desc, txt;


TXT
---
Z  
z  
Y  
y  
B  
b  
A  
a  

Another option is to use Linguistic Sort:

SELECT COL FROM TABLE ORDER BY NLSSORT(COL, 'NLS_SORT=GENERIC_M') DESC

Output

Z
z
Y
y
B
b
A
a