Fish breathe oxygen, o2, dissolved in the water. since water is a polar substance and oxygen is a nonpolar substance, how can there be any oxygen, o2, in the water?

Oxygen gas enters water through Diffusion (movement of gas from higher concentration to lower concentration). The amount of oxygen in water depends on many factors. Like at Higher temperature there will be less amount of Oxygen in water and vice versa. And it is found that at 0 °C, 14.6 mg/L and at 20 °C, 7.6 mg/L dissolves.
The interactions between Water Molecule and Oxygen Molecule are London Dispersion Forces. The polar water molecule induces the polarity in oxygen molecule. In this way the oxygen gas is available in water.