Add the vinegar, mustard, sugar, oregano, garlic powder, and salt to a bowl. Stir well with a whisk.
Slowly add the olive oil while stirring constantly. (This will emulsify the dressing keeping it from separating and making it thick and creamy.) Stir until all of the oil is incorporated. Serve over a Greek-style green salad.
Notes
Be sure to add the oil slowly stirring the whole time. This will emulsify the dressing and ensure that it won't separate.
The Greek dressing will keep well in the fridge without separating for up to 1 week.