Hypertension is one of the major risk factors of cardiovascular diseases. It is also important for elderly hypertensive patients to strictly reduce their blood pressures to prevent cardiovascular events. Although angiotensin II receptor blockers (ARBs) are increasingly used in antihypertensive treatment recently, few studies have been performed in Japan to assess the difference between high-dose ARB monotherapy and combination therapy of ARB with calcium channel blocker (CCB) in prevention of cardiovascular diseases for patients whose blood pressure is not well controlled by ARB monotherapy. OSCAR-study is a multicenter, active-controlled, 2-arm parallel group comparison, prospective randomized open blinded end-point (PROBE) design study. The dose administered is olmesartan medoxomil 20mg/day as ARB monotherapy in the 'Step 1' period. If the blood pressure is not adequately controlled and treatment is well tolerated then the dose is changed to olmesartan medoxomil 40mg/day in the high-dose ARB monotherapy group, or olmesartan medoxomil 20mg/day and a CCB in the combination therapy group in the 'Step 2' period. At least 500 patients will be enrolled in each group, and the follow-up duration will be 3 years. The primary objective is to compare the incidence of a composite of fatal and non-fatal cardiovascular events, and all cause mortality between the two treatment groups.