Synthetic lethality (SL) underlies the success of PARP1 inhibitors (PARPi) in treating homologous recombination (HR) deficient cancers, but extending this paradigm to other DNA damage response (DDR) deficiencies has proven challenging. We performed an in vivo CRISPR screen to identify DDR gene mutations that both enhance tumorigenesis and confer sensitivity to PARPi. Our screen identified FANCA deficiency as a driver of PARPi SL that was validated across diverse human cancer models. FANCA deficiency does not impair HR but disrupts Okazaki fragment maturation (OFM), causing lagging strand gaps and RPA exhaustion upon PARPi treatment. These effects require FANCA interaction with FEN1, independently of its canonical role in interstrand crosslink repair. We find FANCA-mediated FEN1 recruitment is required for OFM at oncogene-associated R loops during PARPi treatment. These findings establish a novel and non-canonical function for FANCA in FEN1-mediated OFM that can be leveraged for PARPi synthetic lethality in FANCA-mutant cancers.