Glad your transmission fix wae easy. I get O2 codes a couple times a year for the last two years. I have been clearing them by disconnecting the battery, or now, with my laptop software. Now that I can see the readings on the laptop, the ranges are well within spec, they may sometimes just throw odd voltages, which, in turn, triggers the check engine light. Yours may be throwing a code because of the water or because it was sitting so long. Clear it and if it doesn't come back too soon, there may not be a problem. There are four O2 sensors at about $100 a piece.
If they remain a problem, do bring it to Flagpole. Those guys are great.