Arrange oven racks to upper and lower-thirds of oven. Preheat oven to 450˚F.
Arrange corn tortillas in a single layer on 2 rimmed baking sheets, 6 per sheet. Brush one tortilla on both sides with oil; repeat with remaining tortillas.
Invert an oven safe wire rack over the tortillas on each baking sheet. Bake on upper and lower-third oven racks for 4 minutes. Switch and rotate each baking sheet and bake for 4 minutes more, or until crisp and golden brown. (Note that the tortillas will continue to crisp once they're out of the oven so keep a careful eye to not overcook.)
Remove from oven and let cool. Serve as desired.
Use tortilla shells made simply with corn, lime, and salt - no preservatives.
Store leftover tostada shells at room temperature in an airtight container or resealable plastic bag for up to one week.