Most tour operators provide you with a heavy parka with a fleece that can be worn separately. It's waterproof with a hood. I've used mine since when showveling snow. It's great, as it's waterproof. Most tour companies also allow you to 'borrow' waterproof boots for all your wet landings( all your landings are wet ), but you will have to provide your own waterproof pants, which you MUST have if you want to stay dry.
Staying dry is the most important thing, as that causes most discomfort. Most people on our trip had gloves, but they weren't wateproof, so when they needed to take pictures from the zodiak, their hands quickly became cold and difficult to use. I recommend that you visit a tackle shop and purchase a set of waterproof gloves with the inserts that are used for surf fisherman. These gloves have the index finger split, so it can be peeled back for casting. While you're not casting, the ability to peel back the index finger is great for keeping your hands dry and for using the index finger to snap your pictures.
Other than that, regluar winter weight clothes will be sufficient. Our tour operator recommended thermal underweal, which we had and didn't use, but that's up to you. we found Antarctica to be warmer when we went( early January ), than it was in New York when we left.
You should take sunscreen and you may want a thermal mask when and if they take you on zodiac rides. You should also take a bathing suit, as we went swimming at Deception Island, but you may not go with OAT's trip.