I would say whatever you want, within the bounds of good taste. When I was doing reserve officer recruiting, I probably swore in 25+ DCO applicants, many of them prior service, and I can only remember one who chose to wear a uniform. And the one who did went the whole nine yards. He bought choker whites and a sword, and we did the ceremony on the gun deck of the USS Constitution. There was an after ceremony reception/lunch at the Harvard Club. At first when he told me what he wanted to do, I thought that it was a little over the top, but I was probably subliminally contrasting to what I normally did with DCOs- raise the right hand and repeat the oath on the Reserve Center quarter deck with the American flag in the background, take a few photos and then sign the papers in my office. When all was said and done, it turned out to be a pretty cool day.