Security researchers shed new light on Duqu, a worm that is believed related to Stuxnet. Russian IT security researcher Igor Soumenkov characterizes the developers of Duqu as “a rather professional team of developers” Duqu was compiled with a 2008 version of a Microsoft Visual Compiler using special options known as /O1 and /Ob1, most likely written with a custom extension to C called OO C. Duqu’s authors appear to be reusing older code written by top ‘old school’ developers.”]
Source: https://www.cuinfosecurity.com/experts-shed-new-light-on-stuxnet-kin-a-4618