Learn Polish: W południe jem obiad.

When telling time by the clock (like "at one o'clock"), Polish uses o. However, for specific parts of the day like noon (południe) or midnight (północ), you use the preposition w.

Why doesn't the word obiad change its ending here? Isn't it the direct object?

You are right that direct objects take the accusative case! However, obiad is a masculine inanimate noun (a masculine noun that is a thing, not a living person or animal). For these types of nouns, the accusative form is identical to the basic dictionary form.

Does południe mean anything else besides "noon"?

Yes! Południe means both "noon" (the time of day) and "south" (the compass direction). In this sentence, the context of eating a meal makes it clear we are talking about time.

Jem means "I eat", but why doesn't it end in -ę like many other verbs?

The verb jeść (to eat) is slightly irregular. While many Polish verbs end in -ę for the "I" form (like piję for "I drink"), jeść belongs to a smaller group of verbs that end in -m for the first person singular.