Heat the oil in a medium skillet over medium-high heat. Add the onion and cook, stirring, until slightly soft, about 4 minutes.
Add the turkey and cook, crumbling with the back of a spoon, until no trace of pink remains, 5 to 7 minutes. Stir in the chili powder and ½ teaspoon salt.
Spoon the filling into the taco shells and top with the lettuce, Cheddar, and tomato.