Shaders are enabled and configured with the
F2 key while in game.
If you're following a mod list: this shader's cloud and sky replacements need to be disabled (they don't fully interact with particles, causing a jarring visual effect. This is a limitation with OpenMW's current shaders system). In the settings menu for this shader, disable:
If you're using the
godrays shader from OMWFX Shaders, then you'll want to disable the sun in this one (set
better_sun to "off").