303, using an O/U in olympic competition has nothing to do with accuracy. Shotguns are not "Accurate" as such, they spread the shot so that your target flies or runs into a cloud of shot or "pattern" hence the nickname "Scatterguns".

The main reason for O/U over S/S is the single sighting plane. With superimposed barrels, you get a less restricted view with both eyes open so there is less chance of blotting out the target, particularly with rising birds. Clays are small and fast so you need to pick them up quickly, O/Us and Single barrels help in this respect. I use O/U and S/S both for clays and game, I was brought up on S/S shotguns and have always found it easier to hit crossing birds with a S/S, probably no scientific reason, just preference and what I'm used to I guess.

In relation to using slugs in Shotguns, you MAY find the O/U more "Accurate" in that you have a single sighting plane and therefore it is easier to "Aim" the shotgun as you would a rifle, both barrels being in the same plane as the foresight bead. With a S/S the barrels are adjacent to the sighting plane and so are a little to the left and the right of your sight line.