How to output oracle sql result into a file in windows?

Use the spool:

spool myoutputfile.txt
select * from users;
spool off;

Note that this will create myoutputfile.txt in the directory from which you ran SQL*Plus.

If you need to run this from a SQL file (e.g., "tmp.sql") when SQLPlus starts up and output to a file named "output.txt":

tmp.sql:

select * from users;

Command:

sqlplus -s username/password@sid @tmp.sql > output.txt

Mind you, I don't have an Oracle instance in front of me right now, so you might need to do some of your own work to debug what I've written from memory.

spool "D:\test\test.txt"

select  
  a.ename  
from  
  employee a  
inner join department b  
on  
(  
  a.dept_id = b.dept_id  
)  
;  
spool off  

This query will spool the sql result in D:\test\test.txt

Very similar to Marc, only difference I would make would be to spool to a parameter like so:

WHENEVER SQLERROR EXIT 1
SET LINES 32000
SET TERMOUT OFF ECHO OFF NEWP 0 SPA 0 PAGES 0 FEED OFF HEAD OFF TRIMS ON TAB OFF
SET SERVEROUTPUT ON

spool &1

-- Code

spool off
exit

And then to call the SQLPLUS as

sqlplus -s username/password@sid @tmp.sql /tmp/output.txt