Oreos twisting in an aerobics class and belly dancing Ritz crackers are two of four spots breaking today for Nabisco Biscuit Co.'s new reduced-fat versions of its best selling cookies and crackers. The estimated $15 million campaign, handled by FCB/Leber Katz Partners, New York, features the tagline "Less fat. Loads of taste. Lots to choose from." The 15-second spots run back to back as 30-second segments.