The obvious theme shared by "The Ones Who Walk Away from Omelas" and "The Lottery" is that of scapegoating. Both stories describe apparently idyllic societies which come to seem sinister when it is revealed that the prosperity and pleasure of the majority depends on cruelty towards one of their number. In "The Lottery," it is one person every year, selected to be stoned to death. In "The Ones Who Walk Away from Omelas," it is a single child, condemned to live in misery and squalor.
It is perhaps a little too obvious to talk about scapegoating alone. However, a good thesis statement could focus on the people's acceptance that scapegoating is necessary. One example might be the following:
Both "The Ones Who Walk Away from Omelas" and "The Lottery" convey the message that most people delude themselves and refuse to face the truth in order to cope with the injustice of society.
One of the most important points to cover is that in neither story does anyone have the slightest rational explanation for why the cruelty is necessary. It is simply asserted that the prosperity of the community rests on this atrocity. You should therefore think about why people accept the proposition in each story without evidence or reason, and what type of person is most likely to accept this proposition. You should also consider the range of opinions expressed about the lottery in Jackson's story, compared and contrasted with the absolute rejection of Omelas depicted in the final paragraph of Le Guin's.