Oil paint by itself has a richness and thickness which provides texture to a painting. But that texture can be enhanced and added to with a variety of tools and techniques. This picture shows a few of the many tools available for this purpose.
Shaping and lifting tools help you to add paint and take away paint. I also sometimes use plant materials. I have found success with certain types, such as ferns, salvias, and small sticks.
When evaluating your painting, look for the texture to match the subject. Old barn wood is not smooth, grasses grow in different directions, clouds are often soft and puffy, and rocks can be hard and smooth, or jagged and rough. Capturing these textures on canvas can communicate more of the essence of the place.
A few examples of creating texture I have used are: salvia leaves for the density of plants on the forest floor under the redwoods, sponges for distant trees, lifting tool for fence lines, toothbrushes to make splatter representing distant flowers on a meadow and a palette knife for smooth rocks.