This is the 'secret' Greek dressing recipe from the pizzeria that I work at. The recipe makes almost a gallon but can be scaled down easily. It can be used for picnics and travels very well, since it doesn't need to be refrigerated. This is the best dressing I have ever tasted, people offer to buy it constantly, but if we sold it we wouldn't be able to make enough to use in the restaurant!
In a very large container, mix together the olive oil, garlic powder, oregano, basil, pepper, salt, onion powder, and Dijon-style mustard. Pour in the vinegar, and mix vigorously until well blended. Store tightly covered at room temperature.