The Oil Brush was designed to imitate painting with an oil brush and paints strokes with a relief. The density of the stroke varies across the entire line. Press B to quickly access this tool.
The tool's parameters will be shown in a floating box over an image (by right-clicking the mouse) and also in the Settings Panel.
To change a parameter, enter a numerical value in the box for that parameter and press Enter (Mac: Return) or move the slider.
Paint color is set through the Color/Swatches palettes. To choose a different color bring the cursor over the spectral bar (the cursor will take the shape of a pipette) and click on the desired color or double-click on the square and choose a color from the Select Color dialog. In some cases, selecting a color is more convenient than manually selecting the components of a color via the sliders.