The CalMac ferry terminal is where the logic of Oban's existence becomes clear. From this pier, Caledonian MacBrayne ferries depart throughout the day for Mull (45 minutes), Lismore (50 minutes), Colonsay (2.5 hours), Coll (2.75 hours), Tiree (3.5 hours), and seasonally for Islay, Jura, Colonsay and Barra. Oban is the hub of the Inner Hebrides ferry network in a way that no other mainland town is.
The experience of watching ferries come and go from the pier — the loading of vehicles, the crossing of the sound, the islands visible on the far side in various states of cloud and light — is in itself a reason to sit at the harbourfront for an hour. If you have any flexibility in your itinerary, book a crossing to Mull or one of the smaller islands: the view back to Oban from the water, with McCaig's Tower on the hill and the distillery visible at the foot of it, is one of the better views of any Scottish town from the sea.