ol.style.FillPattern set fill pattern style to vector features. It defines a set of cartographic patterns to use in your maps.
Hatching can be defined with a size for the lines, spacing interval and an angle.
Cross, square, tile, dot and circle patterns can be defines with a size and spacing interval.
The angle option is a boolean (0|1) for 45° pattern.
You can use font to create a pattern (like FontAwesome patterns in the example ). NB the font must be loaded before pattern creation.