@Chine: I did my triple blood test and Oscar test 3 weeks ago.
Basically, from what I recall when my gynae went through the report with me was that the blood was drawn to test for HIV, STD, Haemoglobin count, german measles antibodies,and other levels like certain hormones etc.
Oscar test will return a risk ratio of 1: X,where X will depict the number of cases of Down Syndrome based on your age group.
Another more important thing about the test is also for the detection of Trisomy 18, which unlike Down syndrome, is usually fatal, with most of the babies dying before birth and those who do make it to birth typically living only a few days. However, a small number of babies (<10%) live at least one year. Again, this is depicted using a ratio of 1:X.