Brian, why would anyone believe that (that Oswald brought a long object other than a rifle)? If Oswald had brought a long package other than a rifle, wouldn't he have said so? Why would he have lied about that? It wasn't against the law to bring curtain rods or other long object to work, was it?
A good thing is to put yourself in Oswald's situation and ask what you would do.
If you brought a long object to work, would you lie about it to police? Would you instead tell them that you brought a small lunch?
You wouldn't? Then why would you or anyone think that Oswald would?
Now, I realize that some will say that it's because he brought a rifle.
But, if he didn't bring a rifle, then he wouldn't lie.
Therefore, there are really only two choices:
Either he brought a rifle and lied, or he didn't bring a rifle and he told the truth.
That's it. That is all the options there are to the rationally minded.