I believe if you go to the ASCO website there is an "OAT registration" link there on the left-hand side. If you fill out the application and make a credit card payment, they send you information via snail mail and email. Normally it takes about 2-3 weeks to hear back via some sort of mail with information to go ahead and pick a time and date for the exam, through Prometrics testing company.
Hope that helps and good luck.