One of the engineering courses I took went into some detail about this. It boiled down to they were still in acceptable parameters but the engineers still didn't like it due to the cold temperatures and the effect they could have just in case. One particular engineer (a contractor) really strongly opposed it due to o rings and ambient air temp day of launching. Management, however, wanted a go ahead and went ahead with it. Again, still within (at the time) acceptable parameters, albeit on the extreme ends, i.e. second coldest launch day. Nasa sums it up quite well here. http://er.jsc.nasa.gov/seh/explode.html

As to a single person Ok'ing the launch, it went through multiple engineers and then managers who all approved it. It was a contractor that actually spoke up about the o rings. Here is a little about the engineer who said don't launch. http://www.nasa.gov/centers/langley/news/researchernews/rn_Colloquium1012.html