We do same in our enviroment. Following is our case
1) out of 100% tables in database, we need just 20% tables to show on website.5% are to be updated back to oracle.
2) We create similar data tables in mysql.
3) we set up cron job in php script (on web interface) (1 script per table). this script connect to both mysql and oracle
3a) small tables are delete completely and all records are updated from oracle to mysql
3b) large tables are checked for update or insert and only those records are updated or inserted to mysql
3c) same way some table are updated from web and oracle depending on last update
4) we schedule all cron script to run at night (once in 24 hours)
5) at last we run one more php script which compares number of records in both databases, if any mismatch found in any table mail is sent to administrator.