1. Allowing direct access to your DB from the outside is a security risk unless you really know what you are doing. I don't recommend it.
2. Writing a web service in a language such as PHP or Ruby on Rails that accesses the DB for you is usually considered the best idea. But there are security concerns here as well.
Since you are using Oracle, I'm assuming there might be critical data in your DB. If your DB contains any serious data, I would recommend you learn how to write a secure web front end before you consider accessing it from your iPhone app, or you could open up your data to unscrupulous users. Or, even better, have someone who knows how to write a secure webservice make one for you to access.
On the other hand, if your DB contains game high scores, go look in the tutorial section for how to access high scores from PHP.
BTW, JDBC is a technology your web server would use to access the database. You will still need a webserver like Apache.