The most basic definition of a "displaced person" is someone forced to leave his home or country. As such, the phrase applies to almost every character in the story. Most, if not all, of the characters experience displacement within the story. Of course, the official Displaced Person is Mr. Guizac, who fled Poland with his family during or shortly after World War II.
Mr. Guizac's displacement and subsequent death seem to represent the displacement and death of millions of people around the world during World War II. When we consider the fact that O'Connor is writing from a Catholic viewpoint, we can see that Mr. Guizac is meant to make us think of Christ. That is not the only way to see Mr. Guizac, but this connection is one that O'Connor wants us to make. Here's a quote from O'Connor's essay, "The Fiction Writer and His Country" that makes her position clear:
I see from the standpoint of Christian orthodoxy. This means that for me the meaning of life is centered in our Redemption by Christ and what I see in the word I see in relation to that. I don't think this is a position that can be taken halfway […]. (source: Collected Works, 804-805)
Now, hold that in your mind while you remember what Mrs. McIntyre says to Father Flynn about Jesus:
"As far as I'm concerned,[…] Christ is just another D.P. [Displaced Person]" (3.23)
Ironically, O'Connor uses Mrs. McIntyre's statement of disbelief in Jesus to help us see that she means Mr. Guizac to be a Christ figure. This is just one of many "hints" about what Mr. Guizac is supposed to represent. In fact, before Mrs. McIntyre became disgusted with Mr. Guizac she said, "That man is my salvation!" (1.58). All this makes more sense when we read what O'Connor has to say in her essay "The Catholic Novelist in the South":
The Catholic novelist in the South will see many distorted images of Christ, but he will certainly feel that a distorted image of Christ is better than no image at all. (source: Collected Works, 859.)
As we discuss in "Writing Style" distortion is one of O'Connor's key fiction-writing techniques. It's natural that she would want a "distorted image of Christ" as the centerpiece of her story. The persecuted and executed Mr. Guizac comes to stand for all displaced persons, in the way that Christ is thought to stand for all sinners. Think about it like this: if Christ is thought to have died on the cross to redeem of humans of their sins, Mr. Guizac died beneath the tractor wheels to awaken the readers to a deeper understanding of the plights of displaced persons everywhere, including Mrs. McIntyre.