In a large skillet, saute ham and onion in oil until onion is tender. Add cheeses; cook 3-4 minutes or until melted. Remove from the heat; set aside.
Unroll crescent dough and separate into 4 rectangles; seal perforations. Place 2 tablespoons ham mixture in the center of each rectangle. Starting with a short side, fold a third of the dough over filling. On the other short side, bring both corners together in the center to form a point. Fold over to resemble an envelope. Pinch seams to seal.
Place on an ungreased baking sheet. Bake at 400° for 10-12 minutes or until golden brown.