Both Olmesartan (OLM)/Amlodipine (AML) combination and Hydrochlorothiazide (HCTZ) have proven to be efficacious and safe in lowering blood pressure, but may not always be sufficient. This study is to test efficacy and safety of the combination of OLM/AML and HCTZ in hypertensive patients whose blood pressure is not adequately controlled with OLM/AML alone. The following treatments will be included in the trial: OLM 40mg/AML 10mg; OLM 40mg/AML 10 mg/HCTZ 12.5 mg; OLM 40 mg/AML 10 mg/HCTZ 25 mg. The trial has four periods. The treatments that will be used are as follows: