Why does one of my breasts look oblong at 12 days post op?

I agree that implants generally look better as time goes on, but they should look fairly good right away. I also think that significant asymmetry is never normal after surgery - assuming that there was no asymmetry before. My guess is that the implant that looks oblong will need a pocket revision because the pocket is too narrow. Talk to your surgeon or seek a second opinion.

No one expects to look asymmetric after a breast augmentation. It is uncommon but not unprecedented. It is possible that the pocket might not be ideal, but implant shape changes as it settles in place, especially in a sub muscular placement. Talk to your surgeon. Express your concerns. Be patient. 

There are many reasons for having to differently shaped breasts.  Which one applies to you depends on seeing your pictures, including what you looked like before, and knowing more about the surgery (type of implants, size, incision site and implant placement)