Fill a pot with lightly-salted water and bring to a boil. Stir in the shell pasta and cook until the pasta is tender but firm to the bite, about 11 minutes; drain.
Stir the peas, ham, Cheddar cheese, onion, mayonnaise, Ranch dressing, vegetable oil, lemon juice, salt, and pepper in a large bowl; add the cooked pasta and stir to coat. Cover and refrigerate 1 hour, or until completely chilled, before serving.