I have been on Oasis and it is the most stable ship I have ever encountered.
I really encourage you not to think too much about getting seasick - I don't think Oasis will rock very much - that ship is a rock. I was on for a week and didn't even feel it move.
Secondly - and this is hard to fight but it is well understood by those of us who cruise a lot - the motion of the ocean is no worse than a flight or a car trip, so a lot of the feeling is caused by you focusing on it. Most regular cruisers are at the point where they don't think about the motion and the don't get sick. We know ships move, but it doesn't translate to feeling seasick.
Trust your medication and the fact that you are on a very big ship and you will be fine. I have crossed the North Sea on ships that are .02 the size of Oasis and the medication I took was enough to keep me from getting seasick. I know you will be fine.