How would you describe the conflict in the story, "The Ones Who Walk from Omelas"? Is there a protagonist and an antagonist? Who are they?
The conflict in this story is a philosophical one. The conditions in the community of Omelas where smiles have become extinct since the residents are always content are predicated upon the tribulations of one miserable child.
"The Ones Who Walk Away from Omelas" is an allegory that examines William James's theory of pragmatism or the practical application of ideas. This theory is associated with "the greater good."
[This theory] considers the meaning of ideas and the truth of beliefs not abstractly, but in terms of the practical difference they can make in people’s lives. (Internet Encyclopedia of Philosophy)
The child bears the burden of all the selfish desires of people who would allow certain conditions so that the majority can be content and comfortable. From this perspective, then, the child is the antagonist. "It" (as the child is called in the narrative) represents the challenges to the conscience with which the individuals are faced when they become aware of the existence of this being whose "abominable misery" is what allows the "all the goodness and grace of every [other] life."
It is the custom of Omelas to expose the "young people" to this malnourished, neglected, feeble-minded, and fearful being. When these young people, who act as the protagonists in this story, are faced with the conditions under which this miserable being lives so that they can enjoy life in Omelas as they do, they must accept or reject this evil. While some accept this evil as a necessary factor for the greater good, others walk away alone "out into the darkness of the fields," and they do not return.
The original question had to be edited down. I would say that the conflict in the story rests in the choices that individuals make. The level of choice that an individual makes is where there is conflict. On one hand, the idea of remaining in Omelas knowing at what cost one's happiness is derived is part of this choice. On the other side of the equation exists what happens when individuals fully understand the implications of their own happiness in terms of needing to leave, to become "the ones who walk away" from Omelas. It is here where I think that there is a clear conflict present. One side of the dilemma displays individuals who find it acceptable to have their happiness come at the cost of this child. The other end are the individuals who either cannot live with this situation or find themselves needing to assuage their own guilt by leaving Omelas. It is this particular choice that Le Guin forces upon the reader, who must wrestle with what they would do and how they function in their own social setting with paralells to the situation offered in the life of Omelas.
I would describe the main conflict in this story as one existing between character and society. The narrator tells us that all young people are informed of and offered the opportunity to go and see the miserable child upon whom all their happiness rests. Many feel anger at the injustice of the child's horrible existence, and a few choose to leave the community and never return, either then or even later, after they've become adults and had more time to consider the situation.
Each of the citizens of Omelas, to my mind, fills in as the protagonist while society, a society which has tacitly decided to continue with this arrangement, becomes the antagonist. When faced with the community's decision to enjoy complete contentedness at the price of one child's abject misery, each citizen must decide whether they can live with this decision and themselves, should they decide to accept the benefits it confers.