Eutrophication is a serious phenomenon in shallow lakes in Hanoi. The most important effect is the change of dissolved oxygen (DO) in the water column. The value of DO in water changed over time, and the variation of DO proved to be more important than the absolute value. The research aimed to assess the role of daily DO variation in classifying the eutrophication status of shallow lakes in Hanoi. Data were collected in eight shallow eutrophicated lakes in Hanoi during 2015–2017 in relation to water quality parameters. A large variation of DO was observed with the maximum value achieved at 15–17 h (180% saturation DO) and the minimum value at 4–6 h (20% saturation DO). The lowest DO value reached 1.5 mg/L, which was much lower than the threshold of survival of some aquatic animals. The daily DO fluctuations correlated with phytoplankton density, the Trophic State Index (TSI), chlorophyll-a concentration and density, and the proportion of DIN:PO4, showing the close relationship between DO variation and eutrophication in the lakes. The decision trees were developed and also selected the ΔDO as the driving variable in classifying the eutrophication status in lakes. The daily DO variation could be an important indicator for eutrophication.