1. Create a .sql file using the below template
-- set the output file in the first SPOOL command
-- add your query in between the SPOOL sections

set echo off
set newpage 0
set pagesize 0
set space 0
 set feedback off
set trimspool on
set heading off
set linesize 555
SPOOL /path/to/your/output/file.txt
select ... 'replace this line with your query'  ;
SPOOL OFF
quit

2. Create a simple shell script

#!/bin/bash
sqlplus oracle_user/password @/path/to/your/sql_file.sql

3. chmod the script

 chmod +x oracle_sql.sh

4. Also to get sql*plus working you mayneed to set a few environmental variables

setenv ORACLE_HOME ..
setenv ORACLE_SID ..
setenv ORACLE_OWNER ..
setenv ORACLE_VERSION ..
setenv ORACLE_CONF ..
setenv ORACLE_BASE ..
setenv TNS_PORT ..
setenv ORACLE_ALERT_LOG ..
setenv TEMPORARY_TS TEMP
 setenv TWO_TASK ..
setenv ORA_LIB ..

5. then just add the oracle_sql.sh to your cron

6. done