Using zester or vegetable peeler, peel thin strips of orange rind from 2 of the oranges, being careful not to include any white part. Cut into wispy thin strips and place in saucepan. Pour in enough cold water to cover and bring to boil (this removes any bitter flavor); drain and set aside. Using sharp knife, cut peel, including any white pith and membrane, from oranges. Cut oranges into round slices and place in glass bowl or in overlapping slices on platter. In saucepan, combine Sugar, water and corn syrup; bring to boil, stirring only until Sugar has dissolved. Add strips of orange rind; simmer uncovered, for 25 minutes or until syrup is slightly thickened. Remove from heat; stir in lemon juice and liqueur. Let cool, then pour sauce over oranges. Refrigerate for 2 to 8 hours, turning oranges once or twice. Taste and add more liqueur if desired.