Oligohydramnios is associated with
Renal agenesis
Neural tube defects
Postmature birth
Premature birth
Etiology of oligohydramnios: 

Intrauterine growth restriction
Post-term pregnancy 
Premature rupture of membrane 
Drugs like ACE inhibitors and prostaglandin inhibitors 
Renal anomalies 
Renal Agenesis 
Infantile polycystic disease 
Upper urinary tract obstructions 
Lower urinary tract obstructions