In Chemistry, why if the oil is heavier than water it goes up from the water if you put them in a glass?

For two miscible liquids, the less dense liquid would flow on the more dense liquid, no matter how heavy the liquids are. Oil is less dense in water, and thus oil flows in water even though oil is heavier than water.

Simply speaking, whether the liquid flows on or sinks in water depends on the mass (or weight) of the liquid per unit volume but not its total mass (or total weight).

Oil is not heavier than water. Put a beaker of oil in one pan of a balance beam and put a breaker of the same size filled with water in the other pan. The side with the water will drop and the side with the oil will rise.

In other words, the density of oil is less than that of water. So the less dense oil rides atop the more dense water when combined in the same beaker.