My program has required OSCE at the end of every semester in school. First couple of semesters was based on a set rubric (introduce yourself and title, identify two patient primers for validation such as name and date of birth, allergies, so on and so forth). Basically first year was "actors" playing patients and you as the intern have to look at their prescription & see what information is missing (if any) and how you would counsel on that specific medication (a top 200 drug).
My last one though (couple days ago actually) was a Drug Information and Discharge Counseling scenario (my last didactic semester at that). This required 30 minutes to look at a patient case that the provider or resident put in, look at current medications, and answer the primary question being asked by the physician (ie how to dose vancomycin, how to dose heparin, what changes of current meds need to be made, etc). You would then be given two follow-up questions and be graded on your recommendations and your follow up responses (either knowing the answer or knowing how to get the answer after putting in your input).
All in all, unless your program incorporates days to "act it out" for practice, you can't really prepare for it. Especially for your first time.
