Place the tri-tip in a large bowl or gallon-sized Ziploc bag and then pour the marinade over it, ensuring the meat is well-coated. Cover the bowl or seal the bag, removing as much air as possible. Refrigerate the tri-tip steak for 4-8 hours, turning occasionally if possible.
About 30 minutes before cooking, remove the tri-tip from the refrigerator and then preheat your oven to 375°F (190°C).
Remove the tri-tip from the marinade, patting it dry with paper towels. Save the used marinade to use as a basting sauce during cooking (basting is optional).
Heat the 2 tablespoons of vegetable oil in a large skillet over medium-high heat. Once it becomes hot, add the tri-tip and sear it for about 3-4 minutes so that each side has a nice sear.
Place the skillet in the preheated oven and roast the tri-tip roast for about 20-25 minutes for medium-rare steak (internal temperature of 135°F/57°C) or 25-30 minutes for medium (internal temperature of 145°F/63°C). Use a meat thermometer to check the internal temperature of the meat.
Every 8-10 minutes during the oven-roasting process, carefully remove the skillet from the oven using oven mitts. Close the oven door to maintain temperature. Using a basting brush or spoon, quickly baste the tri-tip with the reserved marinade, making sure to coat all sides. Return the skillet to the oven immediately. Baste 2-3 times total during cooking.
In a small bowl combine all the chimichurri sauce ingredients. For a smoother consistency, I recommend pulsing in a food processor or using an immersion blender. Be sure to taste and adjust the seasoning if needed. Let the sauce sit for at least 30 minutes before serving to allow flavors to meld and dried oregano to rehydrate.
Remove the tri-tip from the oven and tent it loosely with aluminum foil. Let it rest for 10 minutes on a cutting board before slicing against the grain of the meat and serving.