Pre-heat oven to 375°F; brush both sides of tortilla with oil.
Invert ramekin on a work surface; center tortilla on the bottom of the ramekin.
Gently shape the tortilla down around the sides.
Place a double layer of aluminum foil over the tortilla and press to form the bowl around the ramekin.
Place right side up on a baking sheet and leave about 1/2-inch of the edge of the tortilla above the foil so it gets browned and crispy when you bake it; repeat with the other tortillas.
Place baking sheet in pre-heated oven for 5 minutes.
Remove sheet from oven and use tongs to carefully remove the ramekins from inside the shells.
Transfer sheet back to the oven; bake shells until lightly browned, about 5 minutes.
Remove baking sheet from oven; carefully remove foil from around the shells; reserve the foil.
Flatten foil sheets and invert shells on them; bring foil up just around the bottom of the shells to protect the top edge.
Transfer sheet back to the oven; bake until browned and crispy, 8 to 10 minutes.
Remove sheet from oven and invert the shells once again so they are right side up.
Place on foil and transfer sheet back to oven for the final crisping, 3 to 5 minutes; watch them carefully so they don't burn.
Remove sheet from oven; transfer tortilla bowls to a rack to cool completely.