There is some good shore diving close to Oban itself with the drift round Dunollie Castle being very popular but the more impressive dives are in the Sound of Mull and off the Garvellachs (“or Isles of the Sea form a small archipelago in the Inner Hebrides of Scotland”). In the Sound the wrecks of the Rondo and the Hispania are excellent and the cliff dives at Lochaline and Calve Island being dives that shouldn't be missed.
The Garvellachs are in an area swept by tidal currents that reach their pinnacle in the Corrywreckan the 3rd largest whirlpool in the world which can be heard on the mainland 5 miles away on a good day. Here the dives are down rock faces and pinnacles covered in life often with a current at your back.
Other popular dives include the Breda, a WW2 wreck in Ardmucknish Bay, and the turbulent Falls of Lora at the mouth of Loch Etive - these dives are definitely not for the faint hearted.
There are many other sea lochs close to Oban and they are well worth exploring - particularly if the weather is poor as the lochs are usually protected from high winds. Loch Creran (a few miles north of Oban town centre) offers some interesting and varied dive sites. The conditions are generally sheltered and many dive sites are accessible from the shore.