To float, the 1 kg mass plus the mass of O₂must displace an equal or greater mass of water. So the mass of O₂depends on the density and depth of the water and the density of the 1 kg mass. If you have a balloon full of O₂which is just big enough to make the 1 kg mass float at the surface, and you then pull it below the surface, the balloon will shrink as the pressure increases. Below a certain depth, it will sink.