In English grammar, a preposition is defined as a word that shows the relationship between either a noun or pronoun and other words within a sentence.
Similarly, the object of a preposition refers to a noun, pronoun or noun phrase that follows a certain preposition, completing its meaning.
Since in the sentence provided <em>"Let’s meet Naomi and Sam at ten o’clock", </em>"at" constitute the preposition, it could be said that "ten o’clock" would constitute the object of the preposition.