Is there a tour from Oban to Staffa skipping Mull and Iona, which we will see on our 13 day Scotland tour?
I don't think there's a way without returning to Mull. StaffaTours runs a trip to just Staffa from Fionnphort, but as you probably already know from your trip to Iona, that getting from Oban takes a long-ish ferry journey and getting across Mull. WestCoastMotors has several buses per day to Fionnphort once you get to Craignure.
Depending on you post-tour itinerary, you could get yourself to Kilchoan, and StaffaTours has tours from there, avoiding a return to Mull. Getting to Kilchoan from Oban would involve Mull unless you come from someplace like Fort William.
You should verify that your tour doesn't give you enough time on Iona to get to Staffa.