Yes, only the qa-db-*.php files need to be modified.
Start with qa-db.php and modify all the functions to use the Oracle PHP functions instead. If you start with Q2A 1.6.3 then using the procedural functions like oci_connect will be easier. You could instead grab the latest version from Github which uses the mysqli OO interface - this could be switched for PDO very simply. See here for details.
After you've finished qa-db.php, perhaps check what works and what doesn't, and you'll know what queries you need to change in the other qa-db-*.php files.