Heat oven to 425°.
Season lamb chops on both sides with salt and pepper. In a medium, ovenproof saute pan over medium-high heat, sear lamb chops in oil on both sides. Remove lamb to a plate and lower heat to medium. Add onion and gently saute until softened but not browned. Add Port, orange juice, and vinegar; simmer, stirring, 2 minutes.
Add preserves and stir well. Nestle lamb into pan sauce (adding any juices that have accumulated on the plate) and bring to a boil.
Transfer pan to oven for 15 minutes. Remove meat to a cutting board and let rest, loosely covered with foil, 10 minutes.
While lamb is resting, simmer sauce, stirring, until slightly thickened and reduced to about 2 cups. Place lamb chops on serving plate and spoon sauce over them.