In a sentence of Ouxu, most of the words are can be called nouns. A typical sentence looks like this:
noun noun ... divider noun noun ....
In other words, any number of leading nouns (zero or more), a divider, then any number of trailing nouns (zero or more). (But a lone divider as a sentence in itself makes no sense.) Those before the divider are the "topic", which means information that the listener is believed or assumed to already have a referent for. Those after the divider are new information.
A story might begin, "A woodcutter lived in the forest. One day, the woodcutter set out as usual, early in the morning." In the first sentence, the woodcutter is new information (that's why English says "a woodcutter"). In the second sentence, the referent has been established (now it's "the woodcutter").
Example 4.22. Woodcutter
Pa sioiluia uesa hehe sioitaluffuaape. Belief tree+group+same home+same clause+place tree+cut+person+exist. A woodcutter lived in the forest.
Sioitaluffuaata pa kolehu akosxonpinlihu uesgunpinlikap. Tree+cut+person+agent belief day+indefinite+time early+morning+typical+time home+away+typical+move. One day, the woodcutter set out as usual, early in the morning.
When you're talking, your listener has a mental model of what the conversation is about. And you, the speaker, have a model of what your listener's model is. When your model says that the listener's model includes item X, then X is part of the topic. It doesn't matter how they got to know about X; you mentioned it, or it's plainly visible, or it was already familiar, or they figured it out—no matter where it came from, it's in the topic.
Example 4.23. Topic Examples
Pa uipe. Belief rain+exist. (In a long distance phone call.) It's raining (here).
Uipe pa. Rain+exist belief. (In a face-to-face conversation.) It's raining.
Tenta ligonsatetae pa hoketaun.
They+agent beam+plural+experiencer belief it+overlapping+change-to.
(In Ghostbusters, after "they" and "the beams" have come up.) They crossed the beams.
By using the divider, you are conveying an impression of your model of the other's model of the situation. Nothing forces you to be honest. Often you'll find that a word can reasonably fall on either side of the divider. There is always a difference in meaning between the choices. Other things being equal, new information is emphasized over old information. The emphasis effect of moving a word across the divider is normally stronger than the effect of moving it later in the sentence without crossing the divider.
You can choose different words for the divider, with different meanings. See Choosing the Divider. In this section, I stick to pa, the plainest divider, which means only that the speaker believes what they're saying.
It is also possible to leave the divider out of the sentence. In that case, all the nouns belong to the topic; the sentence may direct attention but it doesn't say anything new. The meaning is similar to "as you can see."
Example 4.24. Omitting the Divider
Uipe. Rain+exist. It's raining (as you can see).
Xitso hanxuae talufha uglaoaoa. Red+attribute wire+experiencer cut+action explosion+avoid. Cutting the red wire (like this) prevents the explosion.
A concept of topic is common across many natural languages—maybe all of them. In some, like Korean, it is grammaticalized, as it is in Ouxu. But Ouxu's concept of topic is not the same as a natural language topic, because it includes all old information, rather than one item that is being focused on.