Congrats on the good score!
About your preparation for the GMAT, did you use any other material apart from the OG? Would you say the OG covers much of the concepts that are tested on the GMAT (except maybe Std. Deviation etc.) ?
Well, about your choice for specialization in the MBA, I suggest you look at Business week rankings etc. for Supply Chain management/operations management. Wouldn't a talk with the people at the business school at Olemiss about your choice of sp. help?
Get with the program!