BACKGROUND: Results in the literature are divergent regarding dissolved oxygen (DO) level to ensure nitrite-oxidizing bacteria (NOB) suppression in mainstream conditions. In this study, a one-stage granular partial-nitritation (PNA) was operated controlling the oxygen transfer over ammonium loading rate (OTR/ALR). The effects of extremely low DO availability on autotrophic nitrogen removal (ANR) performance and NOB suppression were assessed. RESULTS: The strategy applied in the sequencing batch reactor (SBR) in sidestream and mainstream conditions (temperature at 25 °C) allowed micromolar bulk liquid DO concentrations to be obtained. Nitrite production rate (NPR) by aerobic ammonium-oxidizing bacteria (AOB) was sufficient to provide nitrite to anammox bacteria (AnAOB) and sustain nitrogen removal rates (NRR) of 0.34 ± 0.05 and 0.37 ± 0.07 kg N m−3 d−1 during sidestream and mainstream operation, respectively. Extremely low DO assisted by free ammonia (FA; ≤6.2 mg N L−1) in sidestream conditions resulted in complete nitratation suppression. In mainstream conditions, Nitrospira spp. growth led to limited nitratation rates. Decreasing the OTR/ALR resulted in lower oxygen consumption by AOB than by NOB, as well as reduced nitrite consumption by AnAOB, which is likely to be due to granule structure. CONCLUSIONS: Low DO availability did not compromise ANR. Micromolar DO allowed complete NOB suppression only with FA assistance (sidestream) but was sufficient to keep nitratation limited in mainstream conditions.