Also note Air Berlin flies from Dusseldorf and Berlin to several other Greek cities from which you can take a ferry or cheap flight to Santorini, including Thessaloniki. There's typically good availability on MIA, ORD, and JFK flights. Iberia flies Madrid to Athens.
Unfortunately, there's no way to get to anywhere in Greece from Atlanta on any Oneworld carrier with less than two stops, except on British Airways, which seems to be limiting you here.

You'll also find that there are not that many budget carriers flying to Athens from cities that have Oneworld transatlantic service. Easyjet would require an airport change in London, Berlin, or Paris. Your options on Easyjet to Athens are Edinburgh, Manchester, Milan; on Norwegian Air Shuttle, Helsinki; Ryanair - None; Transavia - Amsterdam; Vueling- Barcelona and Rome.

There are actually more budget carriers that fly directly to Santorini, though mostly with seasonal service.
Blu-express: Rome
Condor: Frankfurt,Munich, Dusseldorf
Easyjet: Rome, Milan, Manchester
Edelweiss: Zurich
Jetairfly: Brussels
Norwegian: Helsinki
Thomson: manchester
Transavia: Amsterdam
Volotea: Venice
Vueling: Barcelona, Rome