Here's a link to a good article about olive leaf extract, by Dr. Mercola...
http://articles.mercola.com/sites/articles/archive/2008/09/25/olive-leaf-extract-lowers-blood-pressure.aspx
Here's a snippet from the article; The leaves of the olive tree have been used for medicinal purposes since ancient times, and research has suggested that olive leaf extracts have antibacterial, anti-inflammatory and antioxidant properties. Now it also appears that a supplement containing olive leaf extract could help lower blood pressure and cholesterol.
A study looked at 20 sets of identical twins with "borderline" hypertension -- blood pressure that is above the optimal level of 120/80, but below the cutoff of 140/90 used to diagnose high blood pressure. One member of each twin pair was given tablets containing olive leaf extract, while the other received no supplements but did get lifestyle advice on lowering blood pressure.
After eight weeks, supplement users taking 1,000 mg of olive leaf extract per day showed a substantial dip in their blood pressure overall, and lowered levels of LDL or “bad” cholesterol. The twins who received no supplements showed no significant change in their blood pressure and a smaller improvement in cholesterol.
If you want to incorporate olive leaves as a natural adjunct, I’d recommend sticking with the fresh leaf liquid extracts for maximum synergistic potency. You can also prepare your own olive leaf tea by placing a large teaspoon of dried olive leaves in a tea ball or herb sack. Place it in about two quarts of boiling water and let it steep for three to 10 minutes. The tea should be a medium amber color when done.