Most of the time a dealership's first response will be "replace the oxy sensor" -- it's simple, it makes them a pretty good piece of profit, and it MIGHT work. The light goes off because they reset the system; if it goes back on (which it won't until you're back home), the problem is still there.
Of course if it DOESN'T work it's not their fault, and they go on to replacing the next thing until maybe they cure the problem.
The light can come on from any number of things - as JD said, an exhaust leak; it also can be a cracked vacuum hose, a leaky gascap, a dirty/corroded electrical connection on one of the other sensors, an intake manifold leak, or maybe even another sensor gone flooey.
With the gas-tank overflow, I wonder -- is there any way the charcoal canister can be getting slopped with gas? There might be a sensor there, too - I know there's one on my 'Yota.
Might be worth spending the money on a diagnostic-code reader.