 This study is a double-blind, randomized, placebo-controlled 2:1 niraparib:placebo study in patients with Stage III or IV ovarian cancer. Patients must have completed front-line platinum based regimen with a physician-assessed response of Complete Response CR or Partial Response PR. Additionally, patients must have a normal or 90 decrease in cancer antigen 125 CA-125 following front-line platinum treatment. The study will assess the efficacy of niraparib as maintenance treatment, as measured by PFS. 