This study focuses on the development of a hybrid biological reactor for the treatment of synthetic oilfield produced water. To face increasingly strict regulations concerning produced water discharge, a fixed bed hybrid biological reactor (FBHBR) containing a combination of free activated sludge and a fixed biofilm support was compared to a conventional activated sludge reactor (CAS). After gradual microbial acclimation, a 133-day experiment showed that both bioreactors were able to efficiently remove phenol, toluene, xylenes, and polycyclic aromatic hydrocarbons (PAHs) from a synthetic wastewater with a chemical oxygen demand (COD) removal rate above 95%, at hydraulic retention times (HRT) of 24 h and 18 h, and that only the FBHBR was able to maintain high removal efficiency at an HRT of 12 h. Ecotoxicity tests showed that outlet waters from both bioreactors were non-toxic. Assessment of the bacterial population revealed notable differences between the CAS reactor and FBHBR. In particular, wider diversity was observed in the FBHBR. The marked similarity between the bacterial composition of the free sludge and that of the biofilm in the FBHBR suggests that biofilm detachment played an important part role in bacterial development in the free sludge.