High power conversion efficiency (PCE) and mechanical robustness are key requirements for wearable applications of organic solar cells (OSCs). However, almost all highly efficient photoactive films comprising polymer donors (PD) and small molecule acceptors (SMAs) are mechanically brittle. In this study, highly efficient (PCE = 17.91%) and mechanically robust (crack-onset strain [COS] = 11.7%) flexible OSCs were fabricated by incorporating a ductile oligomeric acceptor (DOA) into the PD:SMA system, representing the most flexible OSCs to date. The photophysical, mechanical, and photovoltaic properties of D18:N3 with different DOAs were characterized. By introducing DOA DOY-C4 with a longer flexible alkyl linker and lower polymerization, the D18:N3:DOY-C4-based flexible OSCs exhibited a significantly higher PCE (17.91%) and 50% higher COS (11.7%) than the D18:N3-based device (PCE = 17.06%, COS = 7.8%). The flexible OSCs based on D18:N3:DOY-C4 retained 98% of the initial PCE after 2000 consecutive bending cycles, showing greater mechanical stability than the reference device (maintaining 89% of initial PCE). After careful investigation, we hypothesized that the enhancement in mechanical properties was mainly due to the formation of tie chains or entanglement in the ternary blend films. These results demonstrate that DOAs have great potential for achieving high-performance flexible OSCs.