Background: Poly(3-hydroxybutyrate) (PHB) is a polymer produced by Azotobacter vinelandii. The production of PHB in a bioreactor is affected by oxygen transfer conditions. The aim of this study was to evaluate PHB synthesis in extended batch of A. vinelandii, to obtain an operation curve (productivity versus oxygen transfer rate) and to scale up the bioprocess. Results: PHB production by A. vinelandii under dinitrogen fixation was evaluated using an extended batch modality under different oxygen transfer rates (OTRs). Extended batch cultures were performed using different agitation rates (400, 600, 800 and 1000 rpm), which determined different OTRs in the cultures. Under the conditions evaluated, it was possible to establish an operation curve of PHB productivity at different OTRs, allowing for the establishment of the maximum productivity of PHB. The maximum PHB productivity obtained was 0.40 ± 0.05 g L−1 h−1, which was reached under an OTR of 22.1 ± 1.5 mmol L−1 h−1. Using the OTR as a criterion to scale up PHB production from 3 to 30 L, it was possible to obtain a similar PHB concentration (7.9 ± 0.0 g L−1) and volumetric productivity (0.43 ± 0.04 g L−1 h−1) on a 30 L scale. Conclusions: The results lead to the conclusion that in extended cultures and under dinitrogen fixation in A. vinelandii cultures, the OTR value determines the maximum PHB productivity. This study provides evidence that the OTR is an adequate parameter as a criterion for the successful scaling up of PHB synthesis for the first time.