Olvance HCT 20mg Tablets 14’s contains Olmesartan 40mg and Hydrochlorothiazide 12.5mg which treat high blood pressure. Hydrochlorothiazide is a diuretic that removes extra water and certain electrolytes from the body. Also, it relaxes blood vessels and improves blood flow. Olmesartan works by blocking the hormone angiotensin therefore relaxing blood vessels. This allows the blood to flow more smoothly and the heart to pump more efficiently.