BACKGROUND: The aim of this study was to evaluate changes in the composition of poly-3-hydoxybutyrate-co-3-hydroxyvalerate (PHBV) by the polyhydroxybutyrate (PHB) overproducing mutant OPNA of Azotobacter vinelandii cultured under different conditions. RESULTS: The oxygen transfer rate (OTR) significantly affected PHBV composition in cultures in shaken flasks; the highest content of 3-hydroxyvalerate (3HV; 28%) was achieved from cultures developed at an OTRmax of 5.87 mmol L−1 h−1. In the bioreactor, cultures were grown under oxygen-limited conditions and different OTRs, varying the agitation rate. The highest PHBV accumulation (85%) was achieved at a low OTRmax (4.96 mmol L−1 h −1), whereas the maximum content of 3HV (8.5 mol%) was obtained at a higher OTRmax (20.3 mmol L−1 h−1). The OPNA strain produced a copolymer of ultra-high molecular weight. CONCLUSIONS: Our results reveal that increasing the OTRmax raises the molar fraction of 3HV in the PHBV copolymer. Therefore, the manipulation of OTR could be a feasible strategy to produce PHBV copolymers with different monomeric compositions at industrial level.