Hi blurbelle, the 20th week detailed scan is supposed to be a compulsory thing in Singapore. Any gynae, hospital or polyclinic you go to for your antenatal visits will definitely have that as a complusory scan. But of course, if you insist on not doing that scan, I don't think any one will force you to.
The 20th week scan is to scan for any structural defects. Structural includes every physical aspect of your baby - such as whether got 10 toes 10 fingers, to all the internal organs, like heart, liver, kidney, brain etc. Whether they appear fine and working well. If any of this structure is not ok, it will be termed as defect or anomaly.
However, some anomaly could be relatively simple and easy to cure, such as a cleft lip. Baby can be operated on to correct the cleft lip easily. So no life-threatening issues. However, some anomalies, especially those that involve the brain, spine and heart are definitely alot more complicated. They often cause alot of other complications which may result in the baby dying in the womb, or not having quality of life if born. As such, docs may advise a termination.
As for OSCAR, it's a test for down syndrome, which is totally different from the detailed scan. Meaning, a well-structured baby with no anomalies, can be born down syndrome. But at the detailed scan, they may be able to pick out the thick neck and slit eyes often associated with Down Syndrome.
For OSCAR, they do a blood test, not a scan. So it's totally different. And OSCAR does not tell you whether your baby has Down Syndrome. It will only give you a risk assessment. ie, whether you are high-risk, low-risk etc. If the result is high-risk, then doc may advise a further test called aminocentesis. they draw fluid from the fetal sac for testing. I think that test is more diagnostic - ie. can tell if baby is DS or not.
However, a word of caution. Nothing is 100% accurate. There were cases where the test said baby is fine, but born with Down's... and vice versa. So, at the end of the day, everything is still pretty much left to God/fate.