Why is oil called more viscous than water when we slip on oil more than we do on water

The quality responsible for oil being "slippery" is called lubricity and it is very different from viscosity- although many oils that have high lubricity are also more viscous than water.

In very general terms, lubricity has to do with the ability of the lubricant to withstand being subjected to extremely high pressures in the gaps between (usually) metal parts that are sliding against one another, without allowing those metal surfaces to actually come into physical contact on extremely small distance scales. The ideal lubricant can form an extremely thin film in those zones of highest pressure which will not rupture, while still possessing a low enough viscosity so as to allow the lubricant itself to be sheared dynamically as the metal parts are sliding.

Since these conditions are often accompanied by high temperatures, the lubricant also has to withstand high temperatures without chemically breaking down or oxidizing. It also has to be non-corrosive to the metals it must be in contact with.

Firstly, friction is a resistive force which acts between two surfaces in contact. Viscosity is fluid friction; the frictional force that acts between the layers of fluid particles or between an object immersed in the fluid and the fluid particles. Scientifically, the frictional force that acts between the layers of oil particles(viscosity of oil) is greater than the frictional force that acts between the layers of water(viscosity of water). The reason why oil is more slippery than water is due to its lubricity. Lubricity is the measure of the degree to which a fluid can withstand high pressures. The reason why you slip is because the oil on the floor cannot provide you with a force great enough to propel you forward. So fluids with a higher lubricity tend to fall you down more, while fluids with a lesser lubricity tend to give you a better chance of walking.