Remove the fell (tissue-like covering) from lamb with a sharp knife. Make several small slits on outside of lamb, and stuff with garlic slices. Sprinkle with salt, pepper, and oregano.
Place lamb, fat side up, in a large roasting pan. Insert meat thermometer, being careful not to touch bone or fat. Add 1 cup water to roasting pan; bake at 350° for 30 minutes or until top of lamb is brown. Add 1 cup water to roasting pan; bake an additional 30 minutes.
Add remaining 1 cup water to roasting pan; arrange potatoes around lamb. Baste potatoes with pan drippings, and sprinkle lamb with lemon juice. Bake 2 hours or until meat thermometer registers 175°. Remove lamb and potatoes to a serving platter.