Are there any oxymorons in "The Lottery"?
No. "The Lottery" is brimming with irony and lots of figurative language, but as far as I can tell, there aren't any oxymorons in the text.
Oxymorons are an interesting little figure of speech: they're a contradiction in terms, or in other words, a statement of two opposing words, phrases, or ideas.
Everyday examples of oxymorons include "open secret," "idiot savant," and "If you want to hold onto something, let it go." Here's an example of an oxymoron from another work of literature: When Juliet calls Romeo a "fiend angelical," it means he's a devilish angel, two descriptions which are, of course, opposite ideas. But this is just after Juliet found out that Romeo murdered Juliet's cousin, so the oxymoron stands: she still loves him (and thinks of him as an angel) but is also very upset (and thinks of him as a fiend).
You can certainly point to some phrases in "The Lottery" that are sharply ironic, such as the idea that "the feeling of liberty sat uneasily on most of [the children]" when they have just been let out of school for the year. The idea that feeling free actually feels uneasy is surprising and ironic, but it doesn't exhibit the kind of precise antithetical contrast it would need in order for us to call it an oxymoron.
Similarly, take this sentence: "He was a round-faced, jovial man and he ran the coal business, and people were sorry for him because he had no children and his wife was a scold." This description of Mr. Summers is ironic: we're told he's got a jovial attitude (a happy, carefree personality) but that people feel bad for him because his wife is so annoying. It's a surprising description, but not contradictory enough to be called an oxymoron.
Again, something like "the proper swearing-in of Mr. Summers by the postmaster" might strike you as funny, since swearing (cursing) is the opposite of proper, but this is the other kind of swearing: promising to tell the truth.
And check out this snarky bit of text:
“Time sure goes fast” Mrs. Graves said.
Ha! Mrs. Graves says that time is going by fast. This kind of dark humor can be found throughout the story, but again, it's not really an example of an oxymoron.