The OAR score is a Composite Score. Like the ACT and SAT and all other standardized tests that rely on composites, each question is given a weight (or value) based on its degree of difficulty. When your test is graded, the total value of the correct answers is summed and you are given your Composite Score. It is different than a percentage. Composites seek to match results to a Bell Curve more accurately than a simple right/wrong results test.
The Bell Curve gives selection boards the ability to see how you differentitate yourself from the pack. Hence, on the ASTB, an OAR score of 40 is considered minimum, but does not translate directly to a "50%" passing grade. A 40 is - according to the Navy - indicative of a higher than average score, though they may have to change that soon.
With the introduction of the computer based ASTB, the sections are now as follows for the OAR:
1) Math Knowledge (27 Questions)
2) Reading Comprehension (25 Questions)
3) Mechanical Comprehension (30 Questions)
More time is allotted for sections 1 and 2 now as well.