Tri tip is a roast that is referred to by several names. It has often been referred to as a Santa Maria roast, from the area in California where it was first widely used.
In a bowl, combine the first 9 ingredients. Cover and refrigerate until ready to use.
With a sharp paring knife, make several small slits in the roast and insert the garlic slivers. Lightly coat the roast with olive oil.
In a bowl, combine the salt, pepper, rosemary and oregano. Sprinkle over the roast.
Prepare a medium hot fire with a kiss of smoke in your grill. When you see the first wisp of smoke from the wood, place the roast on the grill. Close the lid and grill for 40-45 minutes, turning and basting with the refrigerated mixture every 5 minutes, until a meat thermometer inserted in the thickest part of the roast registers 130°F for medium-rare, or until desired doneness.
Let rest for 15 minutes before carving across the grain into thin slices.