Salinity is the primary factor, besides temperature, that can significantly increase the density of ocean water. When the salinity of water increases (due to the presence of salts and other dissolved substances), the density of the water also increases.
While pressure does contribute to changes in water density, its effect is relatively small compared to temperature and salinity in oceanic contexts. Oxygen and pollutants can affect water chemistry and biological processes, but they do not have a significant direct impact on the density of ocean water like salinity does.
So, the correct answer is salinity.
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