You shouldn't need any.
The main reasons for the wide-spread use of oxandrolone in sports is because it is very appealing to female athletes as well as male athletes. It causes little or no virilization properties, demonstrated by its medical uses to treat women. This is rather surprising since oxandrolone does not aromatize either. It's the only steroid that is both safe and convenient without producing excess estrogen. That makes it particularly useful when cutting up for a contest or preventing an increase in body-fat due to estrogenic effects. In fact the main use of oxandrolone to a bodybuilder is in the maintenance of lean mass while reducing body-fat. Oxandrolone itself may not actually reduce body-fat, but it too plays a key role in the process. Like most non-aromatizing compounds it has a repressing effect on the appetite making it easier for the user to control cravings and stay strict with his diet.
Oxandrolone also has little effect on the body's own natural hormone production. The negative feedback was found to be very minor, meaning that during short term use no suppression of Gonadotropin releasing hormone (GnRH, start of natural testosterone production) was noted. This meant that whatever gains made, as little as they may have been, were very easily maintained post-cycle. So there was also no use for products like Clomid or Nolvadex in conjunction with oxandrolone consumption. The easy to maintain low gains would indicate a low binding to the androgen receptor. While not extremely high, it should actually be noted that it does have quite decent binding to the androgen receptor. But the reason for its mild effects is quite likely the low dose used. Rarely if ever are doses higher than 20 mg used on a daily basis. Either because of convenience or due to the high price. But comparing that the doses of other steroids this is remarkably low. So its only logical the gains and side-effects aren't particularly notable.
Oxandrolone does not increase strength through androgenic stimulation, at least not primarily. It stimulates the formation of phosphocreatine, a body compound that can replenish ATP (adenosine tri-phosphate) , the main energy currency of the living organism. This gives an incredible increase in short term anaerobic performance, the type needed for explosive action such as sprinting and lifting weight. This is one of the reasons people stack creatine with Anavar.