It is a positive sign to have DO in water. If there is an absence of oxygen in water, it can be a sign of severe pollution. Sometimes the DO is so low, there is hardly any aquatic life. Most DO in water comes from the atmosphere or plants. The oxygen comes from tumbling water on fast moving rivers that mix atmospheric oxygen with water.
Photosynthesis is another way of oxygen entering water.
DO levels are often highest in the afternoon because of high levels of sunlight that cause plants and algae to photosynthesize rapidly. Because photosynthesis doesn't occur at night, DO levels exhibit diurnal and nocturnal cycles. Many factors contribute to how high or low a DO level may be. Water temperature is one of the causes. Oxygen and other gases dissolve more easily in cooler water than in warmer water.