For information CalMac have a new Islander Priority Plan on the Oban to Craignure, Oban to Coll and Oban to Tiree routes,
This means that only 90% of car space is sold. The final 10% are released a literal 72 hours before sailing (not 3 days, but 72 hours).
Thus if you have been unable to secure space, but don't want to just turn up on standby it is worth checking for capacity at the 72 hour point.
But please bear in mind that the idea of the programme is for islanders to be able to make last minute trips- eg for urgent medical reasons.
Typically this is 6 spaces on the big ships and 3 spaces on the smaller Loch Frisa (to Mull)