It might've occured to Oswald, if he was the assassin that is, to have considered the 5th floor as a more suitable snipers nest location, given that the 6th floor was being relayed and that there might be more workers knocking about that normal. But not enough credit is given for Oswald's deviousness and cunning. He was probably slightly over qualified for the position of order filler, mentally at least.
In terms of the sequence of events on the day and the movements of other workers. It has to be remembered that Oswald was not "one of the lads"..he was a loner, an outsider.Probably by choice at the TSBD. So his movements would not be as predictable as others. This undoubtable helped him keep a low profile on the day. Keep him from being noticed. For this reason and other considerations it doesn't seem all that difficult for him to smuggle the weapon in and get into place to have a chance. Some issues with regard to some of the timings and sightings, before the assassination, but nothing that really affects Oswalds intentions.