This paper explores the influence of oxygen dilution on soot production when oxygen is added to the fuel stream of a steady laminar ethylene flame established over the Santoro axisymmetric coflow burner. Interestingly, at an oxygen mole fraction X_{ax}^{O_2,trans} located between 30% and 32%, a transition occurs as the influence of X_{ax}^{O_2} is suddenly reversed. While the peak mean soot volume fraction increases with increasing X_{ax}^{O_2} within the range below X_{ax}^{O_2,trans}, it is reduced with increasing X_{ax}^{O_2} beyond X_{ax}^{O_2,trans}. To help understand this transition, soot temperature and volume fraction fields are measured by the two-dimensional Modulated Absorption/Emission technique. To assess the sensitivity of the transition conditions, carbon dioxide is added to the coflowing oxidizer stream. Increasing CO2 mole fraction in the coflow, as replacement of N2 in the air, significantly mitigates soot formation in the flame but does not influence the transitional oxygen concentration X_{ax}^{O_2,trans} within the resolution of the measurements. Due to the persistence of the transitional oxygen concentration X_{ax}^{O_2,trans} over a wide range of CO2 replacement of N2 in the coflowing oxidizer stream, it can be considered a distinct characteristic for the assessment of numerical simulations incorporating soot formation and oxidation models. As an original database, the concomitantly measured soot temperature and volume fraction distributions are attached to the present paper as supplemental materials, thus documenting the aforementioned transitions for the whole range of CO2 content of the coflow investigated.