Previous soot studies in counterflow diffusion flames revealed that the sooting limit curve in regions with large oxygen mole fractions (X-O) exhibited marked bending behaviors that indicated a non-monotonic variation of sooting tendency with oxygen concentration. The underlying mechanisms of this bending behavior remained unclear. In this regard, the present study systematically investigated the effect of oxygen mole fraction in the oxidizer stream on the sooting characteristics of ethylene counterflow diffusion flames. We used the near-infrared light extinction technique to measure soot volume fractions of two types of flames that significantly differed in sooting structures: soot formation oxidation (SFO) flames with a fixed fuel mole fraction (X-F) of 0.28 and varying X-O between 0.5 and 1.0 and soot formation (SF) flames with pure ethylene (X-F = 1.0) in the fuel stream and varied X-O from 0.25 to 0.3. We also conducted detailed soot modeling studies by combining the gas-phase chemistry with a sectional soot model that accounted for soot inception, surface growth, particle coagulation as well as soot oxidation. Our experimental and modeling results demonstrated the non-monotonic relationship between soot volume fractions and X-O in SFO flames. A detailed analysis of the evolutionary process of soot formation revealed that the suppression of soot inception and the enhancement of the soot oxidation process with increasing X-O led to a reduction of soot volume fraction. On the contrary, the surface growth rates increased with X-O, resulting in an increase in soot mass concentration. These competing effects led to the non-monotonic variation of soot volume fractions with X-O in SFO flames. On the other hand, in SF flames both the inception and surface growth of soot increased as X-O increased, resulting in the observed monotonic relationship between soot volume fraction and X-O. We also analyzed the soot zone structures and made comparisons between SFO and SF flames. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.