I'm certified Open Water. It's not just a test, it's a class you take over the course of a few weeks. half of the class is classroom and video instruction and the second half is pool diving. The test at the end of the course is a multiple choice written test. The diving portion of the test requires 4 open water dives with an instructor (2 dives (1 tank used=1 dive) per day). In the water you have to show technical proficiency with the equipment as well as navigation and trouble shooting (clearing a flooded mask, losing a mask, buddy breathing, using the 2nd octopus, etc...)
If you have anymore questions, let me know.