I'm looking right now and I can't find any talk of Oswald. I remember when he strangles Dragovich at the end of the game he says, "You tried to make me kill my own president.". Tell me exactly which level because so far NUMBERS, PROJECT NOVA, WMD and the levels after it don't show it. The only thing that I saw was that he said the numbers made him kill Stiener, which could have happened because of the surgery mixed with his will and his meeting with Reznov could have more likely caused. He could have just said the numbers did it because he was insane during the interrogation. Also at the end Mason is seen defending JFK or at least being near one the rings of the CIA's presidential protection strategy when JFK lands, which means he could not have made it to the road where JFK was shot.
Second to last level (Revelations I believe it is called), when you are stumbling through the hallway Mason says the line "Oswald...has been compromised!".
Also, when Mason says "You tried to make me kill my own president.", Dragovich's next line something like "What do you mean, tried" which implies that Mason actually succeeded in assassinating Kennedy. Mason isn't part of the security detail at the end...he is in the crowd to kill him. Hudson even says (again, on the second to last level) that there were several periods where Mason went dark and they didn't know where he was. Well...he was busy assassinating the President before the events at Rebirth Island. Look at the dates on the missions and compare them to when JFK was shot. It all lines up.
Seriously, most of the plot comes together in the second to last level. Just replay that and you'll realize that not only did Dragovich program Mason to kill JFK, Reznov also implanted the suggestion to kill Dragovich, Kravchenko and Steiner.
No CoD to date has had a story this complex. The fact that two people got completely different ideas of what was happening only supports that fact.