Oswald claimed that he was a patsy, but he was an extremely bizarre character who had renounced his citizenship to live in the Soviet Union, and frankly, I would think it unlikely that he acted alone. Oswald probably had accomplices and confidants, and given the direction he was heading, I'm sure he'd probably have try to make a deal--and have it fall flat.
Oswald had a history of lying and was the kind of man who would smile in your face to stab you in the back, so the patsy remark was little more than him trying to worm his way out of getting the blame. He was also not competant enough to get into anything conspiratorial (the KGB threw him out of the country because he was no good to be turned into a spy for them as they were thinking of doing). He also didn't have any friends or confidants or accomplices, largely thanks to anti-social behavior, a history of violence, and his Communism (which he had out of anti-social behavior). The man had to have newsmen carry his coffin at his funeral for goodness sake because there weren't enough family members and almost no friends to carry his coffin, and in the only social organization he was apart of (The Fair Play for Cuba Committee, New Orleans chapter) he was the one and only member.
Oswald had ability (he was a Marine Sharpshooter, and a good one at that, more than capable of hitting Kennedy and more than capable of making a similar shot from twice the distance to boot), psychological reasoning (he had previously tried to kill General Walker and likely viewed Kennedy as the representation of a system he hated, both in terms of politics and the US government, and the wealth of the Kennedy's), and a history of self glorification and violence (Oswald wanted to be remembered for something, which he said himself at some point, which would be remembered hundreds of years later, and wanted to stand out. He was also abusive to his wife and known to have outbursts and fits of rage; again, General Walker).
This is Texas. In the 1960s. Oswald gets the last shock of his life unless some of the nasty things--like JFKs mob connections and the CIA's possible involvement come through.
JFK didn't have mob connections and the CIA was not involved. There's little reason they would be. They pissed Kennedy off on occasion (Bay of Pigs, after which he became angry enough to declare he'd smash it to a thousand pieces and scatter it to the wind), but Kennedy utilized them as much as any other President did in international affairs, and I doubt very much would have dissolved them.
I would imagine that Oswald would probably take a few other people down with him, but unless some of the more outlandish stuff is true he's a dead man.
Again, there's no one but Oswald. He was a social outcast, a lonely man by his own want to be alone (Communist affiliation largely for the sake of being different and standing out, and taking his wife home from the USSR to join him in loneliness), a violent man, and a cold man without remorse. He had no affiliation with anybody in the common sphere, let alone any massive conspiracy or anything of that nature, and had all the ability to kill Kennedy by himself and all the evidence to back up that.
Here's how I see Oswald in Trial: He makes the case a political one and one for him to espouse his extreme leftist views. He claims he is a patsy; that they're only making him the target because of his time in the USSR and his Communism, and uses the trial to drone on about Socialism and Communism and the evils of the American government and all that. In the end, he could either argue Conspiracy (maybe trying to drag down the US government by saying it was the CIA or something), or just arguing that he didn't do it and he's only getting the screws because of his politics. It will be the Kennedy Assassination debate of the last 4 decades incarnate. The prosecution argues that Oswald did it alone, and has evidence to back it up. The defense argues Conspiracy, and has little evidence in itself but can do enough to cast doubt on the prosecution's case.