I actually found a mistake in the manga.
Here, Doflamingo cuts Oars Jr.'s right leg off:
http://www.onemanga.com/One_Piece/555/16-17/Here, Oars Jr. still has his right leg:
http://www.onemanga.com/One_Piece/557/12/
Actually, looking closer at the second picture, the placement of his big toe would make that leg his left one, the one that wasn't cut off.