Learn Dutch: Willen jullie om tien uur vertrekken? - Do you want to leave at ten o'clock?

We use om because it indicates an exact time on the clock. Whenever you talk about a specific time like tien uur (ten o'clock), you use om. It translates to "at" in English, and it is the main preposition you will use for exact times.

Why is vertrekken at the very end of the sentence?

This sentence uses the modal verb willen (to want). In a Dutch sentence, the conjugated verb (willen) takes the normal verb position, and any second verb (vertrekken, meaning "to leave") gets pushed to the very end of the sentence in its full dictionary form.

Does uur mean "hour" or "o'clock" here?

It means "o'clock" here, though uur literally means "hour" as well. When giving a specific time on the clock in Dutch, you just use the number followed by uur, like tien uur for "ten o'clock".

Why does the verb willen come before jullie?

Because this is a yes/no question! Just like in English, Dutch forms yes/no questions by putting the verb first. The subject jullie (plural "you") immediately follows the verb.