What's difference between olmesartan and valsartan?

Very little: They are two different medications in the same class of antihypertensives. They are both angiotensin receptor blockers or arb's.

Almost same : They both are arb, s, both are indicated for hypertension, but valsartan is also indicated for congestive heart failure.

Similar not same: Both drugs are arbs (angiotensin receptor blockers). They are both effective in treatment of high blood pressure and have benefit in patients with abnormal pumping of the heart. However they are different compounds- olmesartan (brand benicar) and valsartan (brand diovan).