<i>…connect to an Oracle database</i>
Since Oracle is the vendor, you need to contact them to ask how to get their driver. The database vendor is always responsible for making drivers available for their products. AFAIK, it can be costly, but I haven’t heard of anyone going that route for a couple years.
One common alternative driver is Oracle’s JDBC Type 4 driver. (A Type 4 driver is essentially written in Java, so it runs on any standard JVM.) Then call Java from your RPG.
I’m not aware of RPG III being capable of calling Java, so you’ll need to create an ILE wrapper that calls Java, and then call the ILE wrapper from your RPG III. Most likely, your ILE wrapper would be written with RPG IV.