Oban, is the gateway to the isles
Although Oban is a lovely place in it's own right, the main reason for tourists to visit is to explore the islands.
From the south pier you can travel to Mull, Coll, Tiree, Bara, South Uist, Colonsay, Lismore and Islay. If that's not enough to satisfy your appetite for islands, you can travel further to Iona, Staffa and a number of other isles from there. Not too far from the south pier you'll find the embarkation point for the ferry to Kerrera. Due to the proximity of Oban to many of the isles, it's possible to enjoy a number of day trips, using Oban as the basecamp, rather than simply passing through.
Caledonian MacBrayne is the Scotland's largest ferry operator and manages all of the services from Oban to the isles. You can find more information about ferry crossings on the CalMac website.