
Oil and Water Effect is an effect which can be used on a layer by selecting from layer styles once you download and save the Layer style file on to your computer.
Once saved, open Photoshop and then use either black or white color using hard brush with 100% Opacity to write whatever you want to.
Then, double click on the layer for layer styles and then go to styles, load the saved ASL file that you have to saved on your computer, choose it and then press OK.
Note: keep the layer blending option to "Subtract". This is for Photoshop CC user, May be used for older Versions of Photoshop (haven't tested on older versions)