Oligohydramnios is the result of either an excess loss of fluid or a decrease in fetal urine production or excretion. In general, oligohydramnios occurs by one of these 4 mechanisms:
1. Amniotic fluid leakage - Rupture of amniotic membranes (ROM)
2. Congenital absence of functional renal tissue that prevents urine production or obstructive uropathy that prevents the urine from entering into the amniotic sac.
3. A chronic reduction in fetal urine production may be secondary to decreased renal perfusion- Chronic hypoxia provokes redistribution of the fetal cardiac output and shunting of fetal blood away from the kidneys to more vital organs.
4. Postterm gestation – after the term there could be decreased efficiency of placental function or decreased fetal renal blood flow and decreased fetal urine production.

Causes for oligohydroamnion could be:
1.Fetal causes - include chromosomal factors, congenital kidney anomalies, intrauterine growth restriction, post-term pregnancy, premature ROM (PROM), and fetal demise.
2. Placental causes - include abruption and twin-to-twin transfusion syndrome (monochorionic twins).
3.Maternal causes - include maternal dehydration, utero-placental insufficiency, hypertension, pre-eclampsia, diabetes, and chronic hypoxia.
4.Drug-induced causes include those due to indomethacin and ACE inhibitors.
5.Idiopathic causes are also possible.

You can't do anything to prevent oligohydroamnion because this condition basically depends upon factors that you can't influence. You can only contribute to the early detection by going regularly to the scheduled obstetrician check ups during the pregnancy.