Olmesartan is a medication used to treat high blood pressure, heart failure, and diabetic kidney disease. It may be used alone or in combination with other drugs to lower your blood pressure. Olmesartan belongs to a class of drugs called angiotensin receptor blockers. It blocks the action of angiotensin II, a chemical in the body that causes the blood vessels to tighten and narrow. This drug helps to relax and widen the blood vessels. This lowers the blood pressure.