Oxygen (`O_2` ) is a nonpolar homonuclear diatomic molecule and water (`H_2O` ) is a polar molecule. When a polar molecule approaches a nonpolar molecule, it can induce a dipole moment in `O_2` by distorting its molecular electron cloud. The permanent dipole of water can then interact with such induced dipoles so that both of them come pretty close and dissolve to a certain degree (see the attached image). At `20^oC` , oxygen has a solubility of about 9 ppm in pure water. With rise in temperature, this value falls off.