ANSWER: Get yourself a lawyer with solid experience in disability retirement cases. If you lost the first round going it alone, you are even more likely to lose this second and final round. It is inherently more difficult to win a legal matter on appeal (reconsideration is an appeal) than it is to win it the first time around. Asking OPM for reconsideration is the same as accusing it of having made a serious mistake and asking it to correct that mistake. I don't know too many people who take kindly to such requests, let alone people insulated by a huge bureaucracy.
Winning on reconsideration requires first that someone really considers your argument and second that your argument meets that person's standards. You will not meet OPM's standards if you merely point out all of the errors they made. Instead, you will have to demonstrate how those errors prejudiced OPM's decision against you, since errors per se are not grounds for reversal. That's tough to do, especially when OPM bureaucrats often see their mission as trying to find reasons to deny disability retirement rather than to grant it. You need a good deal of experience to be successful against this mind-set.