Having rather recently written a report about the J.F.K. murder and what role Oswald really did play, I have my opinion about this matter...
Oswald looking so cool on all the photos of him after his arrest can both make one think he didn't do it, and that he did do it.
If he
didn't do it, he's looking calm because he knows he is innocent (though one would think that someone having been arrested for a police murder AND for murdering the president would look pretty terrified).
And if he
did it, his calmness just supports the picture of his coldbloodedness that the Warren Comission makes such an effort to prove.