Learn Polish: Dużo o tobie opowiadał, ale nie wspominał o niej. - He talked a lot about you, but he didn't mention her.

The preposition o (meaning "about" in this context) requires the Locative case. tobie is the Locative form of ty (you), and niej is the Locative form of ona (she).

I thought the word for "her" was jej or ją. Why does it change to niej here?

In Polish, third-person pronouns (like he, she, it, they) add an extra n- at the beginning when they come directly after a preposition. This makes pronunciation much smoother. So, instead of saying o jej (which sounds a bit clumsy), you must say o niej.

Do opowiadać (to tell) and wspominać (to mention) always use the preposition o?

Yes, when you are talking "about" someone or something. Just like the verbs mówić (to speak) and myśleć (to think), verbs that deal with sharing information or having thoughts regularly pair with o followed by the Locative case.

Why is there no word for "he" (on) in this sentence?

Polish often drops subject pronouns because the verb ending gives it away. The -ł ending on the past tense verbs opowiadał and wspominał tells us the subject is a single masculine "he". Adding on would sound unnaturally emphatic, like you were stressing that "HE talked a lot..." as opposed to someone else.