For understanding the potential of achieving nitritation under different oxygen and ammonia levels, two activated sludge reactors with high (R-H) and low (R-L) dissolved oxygen (DO) were parallelly operated. During over two months continuous operation, rare nitrite accumulation was observed in both reactors. K-strategists Nitrosomonas oligotropha and r-strategists Nitrosomonas europaea were enriched in the R-H and R-L, respectively, yet their response to DO variations was almost identical. Although K-strategists Nitrospira defluvii dominated both reactors, species cultured with low-DO exhibited higher oxygen affinity. Instead of DO, ammonia and nitrite availability should be the key factor for the selective enrichment of these nitrifiers. Taken together, the limiting ammonia for ammonia oxidizing bacteria and the better oxygen-uptake capacity of nitrite oxidizing bacteria was respectively responsible for the failure of nitrite accumulation in the R-H and R-L. This study supported that high DO coupled with excess ammonia would favor the achievement of mainstream nitritation.