It is possible to roast tri-tip in the oven, but it should be briefly seared under the broiler at the end of the cook. An internal temperatures of 125° to 135° in the thickest part yield a nice range of doneness across the roast. Tri-tip is grilled on a grill with a higher and a lower temp side, so it can be moved; lower barbecue temperatures (to the same doneness as above) give good results.
One trick is to not trim the fat until the roast is cooked so that the juices can tenderize the meat. Tri-tip should be sliced against the grain but be careful because the grain direction will change slightly from one end to the other.
Bruce Aidells suggests, preheat the oven to 450 degrees F. Remove the meat from the marinade and pat it dry. Discard the marinade. Place the tri-tip, fat side up, on a rack in a shallow roasting pan. Roast for 20 minutes, then begin checking the internal temperature: You should remove the roast at 115 degrees F to 120 degrees F for rare, 120 degrees F to 125 degrees F for medium-rare.
Transfer the meat to a carving board or platter, cover loosely with foil, and let it rest for 10 to 15 minutes before carving to allow the residual heat to complete the cooking and the juices to stabilize.