Mix together flours and salt on a clean surface, make a well in center, and gradually add 1/2 cup water and oil, working flours into center until a dough forms. (If dough is too dry, add more water, 1 tablespoon at a time.) Knead dough until smooth and it springs back when gently pressed, about 5 minutes. Wrap in plastic.
Sprinkle 2 rimmed baking sheets with semolina flour. Divide dough into 8 pieces; keep covered with plastic wrap. Roll one piece at a time into a long rope, about 1/3 inch in diameter, then cut rope into 1/3-inch-long pieces. Using tips of index and middle fingers, firmly press each piece and pull dough toward you so it lengthens slightly and forms a curl in the middle. Transfer pieces to baking sheets. Repeat with remaining dough.
Cavatelli can be refrigerated, covered, up to 4 hours. (Or freeze on sheet; once firm, transfer to a resealable bag and freeze up to 3 months. There's no need to thaw before cooking.)