High-grade serous ovarian cancer (HGSOC) is common with poor prognosis. Limited therapeutic options are available, and the development of new therapies is of high priority. Using a genome-wide RNAi screening approach, we demonstrate that the combination of the RNA Polymerase I transcription inhibitor (CX-5461) with DNA Topoisomerase 1 (TOP1) inhibition shows cooperative cell killing in multiple HGSOC cell lines. TOP1 inhibitors, such as Topotecan, are currently being used as a second-line therapeutic option for HGSOC. We hypothesise that while acute treatment with CX-5461 inhibits Pol I recruitment to ribosomal DNA, TOP1 inhibition enhances the formation of RNA-DNA hybrids (R-loops) but not other secondary structures such as G4-quadruplex DNA to induce a robust DNA damage response and cell cycle arrest. Combined treatment with CX-5461 and TOP1 inhibitors demonstrates striking efficacy in vivo, which provides evidence for a novel strategy to treat HGSOC patients.