5507 Background: The CLIO trial (NCT02822157) evaluated olaparib single-agent therapy versus standard of care chemotherapy in platinum-resistant (recurrence within 6 months after last platin) ovarian cancer (PROC). Methods: Eligible patients with measurable disease and ≥1 prior line of chemotherapy were randomized 2:1 to Olaparib (OLA) monotherapy (300 mg tablets, BID) or physician’s choice chemotherapy (CT; PLD 40 mg/m² q 4 wks; Topotecan 1.25 mg/m² day 1—5 q 3 wks; Paclitaxel 80 mg/m² day 1, 8,15 q 3 wks; Gemcitabine 1000 mg/m² day 1, 8 and 15 q 4 wks). Primary endpoint was objective overall response (ORR) per RECIST v1.1. Germline BRCA status was available for all patients. Disease control rate (DCR) was defined as response for at least 12 wks. Results: 100 patients with PROC were randomized 2:1 to OLA (N = 67) or CT (N = 33). Median prior lines of treatment was 3 (range: 1—8). ORR (unconfirmed) was 18% (12/67) for OLA and 6% (2/33) for CT. ORR for OLA was 38% (5/13) in gBRCAm and 13% (7/54) in gBRCAwt patients. Of note, 2 patients with gBRIP1 mutation had no response under OLA. DCR was 35.8% (24/67) for OLA and 42% (14/33) for CT. DCR under OLA in gBRCAm was 62% (8/13) compared to 30% (16/54) in gBRCAwt disease. The median duration of response (DOR) and the median progression-free survival (PFS) was similar: 5.4 months vs 4.5 months (DOR) and 2.9 vs 3.4 months (PFS) for OLA and CT, respectively. Grade ≥3 treatment-related AEs occurred in 60% vs 52% for OLA and CT, respectively. Somatic HRR mutation analysis is ongoing and will be presented. Conclusions: Olaparib monotherapy showed a favorable response rate in PROC compared with chemotherapy also in gBRCAwt patients. Analysis of clinical endpoints in relation to HRR is ongoing and will be presented. Clinical trial information: NCT02822157.