We also had an OSCE at my school in July/August after 3rd year.
I didnt take my Step 2 CS until this past February. (And I hadnt really seen any patients on my own since September/October)
I barely studied at all -- No more than a combined 2 or 3 hours I would say, and that was only to read over the First Aid in order to know how the test day goes and to pick up some VERY quick pointers, paying the most attention to things that you ought to say to the patient. I had the same set of technique for all of them -- get most of my history, ask ROS as I washed my hand, VERY brief physical. Then I would tell them I'm going to summarize (which I would). Then the two magic phrases: 1) "Do you have any questions" and, 2) "I cannot yet tell you what the diagnosis is, but X, Y, and Z are likely possibilities and we will do A,B and C to try and help us figure out what is going on so that we can give you the best relief"
My plan had been to study the whole day before at the hotel, but as my flight got canceled and I had to catch a later flight, I didnt get to the hotel until 10 pm the night before, so I gave up on the thought of studying and went to sleep. (And I STILL literally would fall asleep during the write-up portions)
The way I figure, I was bored, the SP's looked bored (minus 1 particular SP I thought was a bit flirty) and I bet you anything they dont remember half of what went on in teh session and so assume you did most things.
Still managed to pass.